lower powered units even from quality brands are very often more expensive than higher powered units. just the way it has always been. demand is lower for these and they can get higher prices for them.
this is why we regularly suggest those 500-550w psu's for systems that may only need 300w or so. it's just cheaper to get a quality 500w psu than a quality 350w one.
the extra power won't hurt the system and the capacity does not get used if it is not needed. efficiency is only effected if you are using a 1000w unit for a 250w system. but a couple hundred extra watt capacity won't have any issues at all.