Which one joke was that 40k is what you're going to end up spending... Try a system that you'll only need 5-10 (or sometimes 1) minis on a side.
There are a number of old west rulesets that I can think of, Shootout, Legends of the Old West, Gutshot, The Rules with no Name, a Fistfull of Dice, Six Gun Sound and others that I'm sure I've missed. Your posse is usually 5 - 8 figures.
Necromunda was a GW product set in the 40k 'verse. You were running a street gang of usually 6. It's out of print now, but I think Two-Hour Wargames' 5150 is the same style.
If you only want to paint one figure, try a gladiator system. And all you need for terrain is a circle. My favorite is Habit Hoc Habit, although Warhammer Historical has just released "Gladiator" (original I know) which I'm trying to get a copy of. HHH also has a campaign element to so you can run a school.
Another thing might be to go to 15mm, or maybe even 6mm. The figures are a lot cheaper that way. Check out The Miniatures Page for more info then you'll ever need.