Cheap Place to Lift Weights?

If you don't mind the walk, the Red Hook Rec Center is your cheapest option. The gym is in the middle of the men's locker room and it has a bit of a "prison yard" feel, but the staff is friendly and the hours are decent.

http://nycgovparks.org/sub_things_to_do/recreation_centers/rc_red_hook.html

A Central Brooklyn Y opened on 225 Altantic (at Court). I haven't worked out there, but the facilities looked nice when I took a tour.

There's an older Y in Park Slope on 9th Street: http://www.ymcanyc.org/sub.php?p=about&sp=prospect/prospectabout

I've belonged to this Y for about 4-1/2 years. They've put a lot of work into it since I joined and--as a result--it's always crowded. Unless you go off peak, it's a complete pain in the ass.

Body Elite (http://www.bodyelitegym.com/) on Court @ Union is decent, if nondescript. Nothing fancy there, just your basic workout setup spread over four floors.