I'd say it depends on the locomotive and the paint scheme you're replicating more than anything else, Chris. Complex paint schemes will cost more, simple ones will cost less.
For example, on the simple end of the spectrum, if you were going to paint a Norfolk Southern diesel, you'd need a bottle of black paint, a bottle of silver and a bottle of yellow. Depending on the type of paint you use, you'd also need some kind of thinner or solvent (or not, if you're using water soluble paints). Then there's masking tape (I use Scotch Magic tape, which is always around my house), newspaper for a dropcloth and for masking large areas, and a cardboard tube from a roll of paper towels for holding the locomotive shell. Don't forget the decals (I use the NS set and the diesel data set for these locomotives) and the Micro Sol and Micro Set, a paper towel, two fine paintbrushes, a sharp knife, and some scissors.
Add all that up and it's a lot of money, but some things are free like the paper towel tube and the newspaper (or at least, you've paid for their original use, the next use is free). Other things only have to be purchased every so often, like the yellow and silver paint, which are used for the handrails and window frames, respectively, so they last practically forever. You'll go through the base color faster, but still I can paint at least a dozen locomotives with a single bottle of paint, and that's over several years. And the decals will do four or five different models, too.
More complex locomotives will generally only require more time, not necessarily more money. Of course, you may have to buy more colors of paint, but you'll use less since you're covering less of the model with each color than a single-dip paint job like NS.
So, if I mentally ring up my average model for just the paint, not the details or the weathering or anything else, and assume that I'm spreading the expense over past and future models (so as not to count the full cost of a bottle of paint, for example), I'd say a dollar for the paint, a buck-fifty for the decals and probably three and a half dollars for general supplies. But don't get any ideas that I'd do any custom painting for six bucks!
