Heck it's my M. O. to buy cheap miniatures and use them in my games. My target price for figs is a buck each (though there are exceptions of course) for line figs and 2-5 for characters etc. Purple Power is your friend at these prices.
If you're talking super cheap for practice...
I'd avoid the toy army men as they haven't got enough detail to really be good options. However, EM4 has a whole range of cheap plastic miniatures in sci-fi, post
apoc and fantasy genres.
http://www.em4miniatures.com/acatalog/FANTASY.html
The cheapest are their plastic orcs and dwarves that were part of the original "fantasy rules" boxed set and are now sold separately and in batches of 50.
Pretty nice figs for 32 cents (25 cents if you buy in bulk) a piece!