The xilinx and altera fpgas are getting down towards the 0.18 0.15 micron range so power consumption is comparable with a normal chip,if you think that the virtex 2 range of fpgas from xilix run at core voltages of 1.8 - 1.3 volts.This is often a good reason for going with custom designs as you can often get the same performance as traditional devices at lower clock speeds.