Before you can tell him how much it will cost, you have to get a better idea of exactly what he wants you to do with it and how much money he had in mind to pay you. As he is your friend, you could of course work in a discount if you liked.
In my former career of graphic design I did commission work a lot. The bane of any pricing negotiation is when the client asks you to provide some sort of estimate but gives you only the most bare bones idea of what they want. It's up to you to get more info out of them, pin them down to details. DO NOT commit to a firm figure if they won't give you enough to create an accurate number from; that's a recipe for either ripping them off because you over estimated or ripping yourself off because you underestimated.
Things to consider:
1: Their budget for it:
Don't offer them more than their planned expenditure dictates.
2: Time and materials:
You need to know how much detail they want and what sorts of detail if you are to properly calculate how much time it will take you to complete it and how much any extra materials you may need to complete it will cost.
3: The value of your time:
You know how much your time is worth to you and that you could be doing a lot of other things with it; make sure the other party knows this from an early stage of negotiations. You will not get much respect in negotiations if you do not show that you value your time.
A lot of people sell themselves short and set themselves up to take a loss because they don't defend the value of their time enough. I think this is a psychological thing as a lot of us probably have a little voice in the back of our heads (likely a parent's voice) telling us not to be "Greedy". Valuing your time is NOT greedy, it's self respect, full stop.
4: Balancing the above three:
More often than not, you will have to tell that potential client that their planned expenditure and the level of detail they are expecting simply are not compatible with each other. It's not unusual at all for a potential client to ask for the moon and the stars at a near slave labour cost.
It's not that they are really out to rip you off; quite often they just have no idea at all what goes on on your end of things, so you have to educate them a bit. Take some time to show them what you do, how you do it, the materials you use and their associated costs as well as the time it takes to achieve the results that you do. After that, it's on them if they want to continue to persue the project with you.
If they want to continue, then you either have to talk them up in price or talk them down in detail. They'll have to decide where the compromise will be.
The cost of your time, unless you're doing the project for friends or charity, should be non-negotiable. you have to draw your line somewhere and, as a service/product provider, this is where you do it. You get no respect from others if you aren't strong enough to show self respect. Just don't forget where self respect ends and conciet and arrogance start
Once you and your potential client strike an agreeable balance, you should be able to carry out the commission with minimum risk of disappointment to you or your client.
Pitfalls to watch for:
1: Your hobby as a money making prospect:
Using arts, crafts and hobbies to make money can be very difficult as a lot of potential clients will just think you're doing it for the joy of it and the creative rush more than the money. That's fine if you have another source of primary income and you just do some piece work from time to time. However, if commission work becomes a bigger part of your income, you have to know when it's time for the hobby hat to come off and the business hat to go on.
My mother loves sewing and embroidery and has done both for years both as a hobby and as a source of money. She does have a normal career to make the majority of her income, but she does do a lot of sewing work for friends. Her work will get admired by friends of those friends and they will want her to do some sewing or embroidery for them. Nearly 100% of the time when she gets approached for work for friends of friends they are shocked at her estimates for the work. She always carefully and patiently explains to them where the costs are incured and so forth and how the estimate breaks down. Quite often the person approaching her for the work will back out.
Lots of people are more than willing to "Ooooh and aaaah" at your work and tell you how much they like it. A lot of the time, if they like it enough to enquire about commission work, they will back out quickly if they can't get it for a price that would amount to the same as them throwing a few coins at some street kid in a third world country selling jewellery made from scrap metal.
You can see a parallel to this situation in our hobby. As modelers, we go on and on about how we would like to see this that or the other kit produced but a lot of us are woefully ignorant of the processes and costs involved in making a new kit. We have in our heads an idea of how much we might be willing to pay for said kit if it ever came to be, but if we were ever sat down by those making the kit and had the whole process outlined for us and had the breakdown of the MSRP explained to us a lot of us might back out of wanting that kit in short order.
2: Legalities:
Of course they don't affect every transaction, especially those between friends, and they differ jursdiction to juisdiction. however, when you do commissions for people you don't know personally or for organisations, ALWAYS, ALWAYS, ALWAYS get eveverything in writing! Have a paper trail to protect yourself with if things get ugly. This is especially important for larger more complex commissions or if you do enough commissions on the year that it could be considered taxable income. Basically, cover your backside. Your potential client might see it as nit picky and bureaucratic of you, but in the long run you're doing both parties a big favour by insisting on it.
Sorry for being so long winded about it, but a lot of people really underestimate all of what's involved and very quickly get in over their heads and into serious financial and even legal trouble doing this sort of thing ignorantly.
I have no idea how deeply you want to get into commission work, but if you want to look at it as potentially serious income, do yourself the favour of signing up for some evening or weekend business courses. Making the product is one thing, but don't underestimate the importance of good basic business skills.
I hope I've been helpful and not put you to sleep.