I take it you live in the country, or in a small town without a hobby shop. Then it becomes tough. There are SO many colors that, as you say, it would cost a fortune to stock up.
If you have a Michaels craft store in the area, they carry Testors enamels- not a super selection, but enough that I get by with that for many models.
You need to learn how to mix paint. If you can do that, you can get by with a lot fewer colors. You need at least white, black, and some primary colors. You might want gloss and flats in each, though you can get by with gloss and dullcoat after decaling. They make flat finish polyurethane that you can get in any hardware or paint store if there is no local place to buy dullcoat.
Go to a library if your area has one and pick up some books on beginning oil (or acrlyic) art painting. Most of these books have a section on mixing paints. Then buy a color wheel or color chart from an art supply place on internet.
One of the big paint buying problems these days is that model building is becoming less popular with kids, so the big chain big box stores like K-Mart, Target, and Wal-Mart no longer carry model kits and the paints for it. So learning to paint by mixing your own colors is becoming a more and more useful skill.