If it can only mine btc, I don't see how it can still be profitable right now to be honest. Some of those Z-Tex boards have 128MB memory, so if I'd even switch to FPGA now, it would have to be at least LTC compatible. Look at some of the people with their Minirigs in the wild threads. Several guys already with 150+Gh/s Minirigs. Not even talking about Asics.
@500mh/s you would make about $3 per day, using 25-30w of power costs about 10-15c per day.
How are you doing your calculations?