Been thinking about this question and have decided it is something of a trick question. Very rarely do I buy something other than the model because I have most of the additional items (Paint, decals, photo-etch etc). In most cases, I buy a bottle of paint (cement, thinner etc) because the bottle I have is getting low. And just because you bought a bottle of paint or cement specifically for that model, do you use the entire bottle of paint on that model? Or does it get used on two or three more models after you've finished the particular model you origionally purchased the paint for?
Perhaps a more apropo question would have been, How much have you invested in your hobby? Try this - take a paper and pencil and sit down at your hobby area and look around. If your house burned down tonight (God forbid!) what would it cost you to replace everything you have that you use in your hobby. The tools, the decals, the paints, the kits you have in your stash, your finished kits, the paints and glues, the scratch building materials, the after market items and the reference materials. Write down each item and then what you figure it would cost to replace them. Then add them all up. The final figure will either amaze or scare you (or both). And then think about those items you have to replace from time to time during any given year (Exacto blades, paint, thinner, cement etc) but aren't for a given project.
What I'm trying to say I guess is that this is a hobby and like any hobby there are expenses we don't think about when we participate in it. As long as you can afford it, then do it. But if you ever get to the situation that you have only enough money to buy the baby some food and Pampers or buy the latest Tamigawa release and the new PE for it and you buy the model, then it's time to step back and take a look at your priorities. And like my wife says, "I don't mind him spending money and working on models. At least I know where he is and the money isn't being spent on booze and broads." (I overheard her tell that to my mother)