Head over to diysolarforum.com and look for the group buy thread (it's not really a group buy anymore, it's ongoing). I picked up four 280Ah cells for right at $400 (exact amount varies a bit on shipping destination). Rumor has it that shipping has gotten pretty slow in the last few months, however, so you have to be patient.

Anyway, that gives you 3.36kW of capacity (assuming a 12V 4S config) for $119/kW. To be fair, it will be slightly more when you price in your BMS of choice.