Polar Ice and Svedka are the two best readily available cheap vodkas. You should try them.
Soonerborn beat me to it. I got a massive bottle of svedka for $10. I had never tried it but I couldn't pass up the bargain. At room temp it's good, chilled and it's almost like water.
Those are good, solid value pics but I prefer Sobieski: http://www.vodkasobieski.com/