Saving money on vodka is a tricky subject because it is very easy to get a really bad and cheap vodka. There are many brands that cost around $10 for a 750 ml bottle and some are considerably better than others. Those listed below, depending on the market, will only cost you about $10.
Most vodkas that go for the $10, like McCormick, Popov and Hawkeye. Unless you use a filtering system of your own like the Vodkastick or want an enormous hangover tomorrow, I recommend you stay away from those. The cost savings is not worth it.