I bought every season on the day of their release and haven't regretted it. In fact, I bought S2 with a Fan Club discount of 20% ... and got a membership pack with it.
Anyway, I've never regretted spending full price on any of the seasons. I can't friggin wait for S4. It's soooo good right now.
But, uh, for prices ... s2 is pretty expensive. I don't think you'll find any great deals on new copies of it ... I'd say the best thing to do is try eBay, or Used DVDs at Gamestop or whatever.