The cheapest 88-note keyboard that you are gonna find is gonna be the Kurzweil SP-88. If you go for the semi-weighted version of it (as opposed to the fully-weighted SP-88
x), you should be able to get it for about $600-700. Check at
www.musiciansfriend.com, they usually have good deals on the SP-series. Another board to check out would be the Roland RS-9... it's got 88 semi-weighted keys and comes at about $1000. The extra cost is justified seeing as the RS-9 has a LOT more variety when it comes to sounds. The SP-88 has mostly piano, electric piano, and organ sounds. The RS-9 has a fairly full soundset all derived from the XV-series.