I have built model's for money in the past. ( mostly railroad stuff)
If you do try your hand at this, I have a few "Pointer's".
! : Dont give the customer a DEFINATE time frame. You can "Sugest" that it will take "such and such day's / week's , but dont tell them "It will be done at noon on day three. Too much can happen ( lost part's , paint run's , bad decal's , ECT.)
2: Know what to charge. I usually charge 3 X whatever the kit and paint cost. ( A $30 kit , and $5 for paint would work out to $105. You wont get rich off your usual 1/25 scale kit's . ( That's why I did 1/87 scale train stuff.... I could "crank out" 6 or 7 build's in a few day's .)
3: Dont take on more than you can handle. I had customer's comeing out of the woodwork ! So I kept upping my price's.... and they just kept comeing ! I finally wound up charging $35 an hour ! But then I spent 100 hour's building a steam crane. There was NO WAY I could charge this poor guy $3500..... so I went back to my first price's , and charged him $250. ( A few year's ago I saw the steam crane I built at a train show, going for $450. Nice to know my stuff was in demand. )
4: Custom order's. I found doing custom work, to be a bit "tedious", and it can suck the life right out of you. I would still do it, but only a few build's during the year. When I just built stuff I liked ( usually in multiple's ) and just offered them up for sale, I got along much better with my customer's . My best seller's were my 1/87 Mack truck's. I could crank out 10 in a 8 hour period , and charged $30 each. ( That's $200 profit in 8 hour's !)
I never charged for watching the paint dry.
5: dont let your hobby, become a "JOB" ! They say, " pick something you love to do and you will never work a day in your life". "THEY", dont know jack squat about it !
I finally got burned out on the whole "for hire" thing . and gave up.
I gotta make "ME" happy first !
Hope this help's . Let me know if there's anything else about "Modeling for fun and profit" that you have question's about.
Tanker-Builder can give you even more "Tip's " ( He's been doing this for a LOT longer than me. )