I see people using 300W PSU for their case conversion...what the heck do they put in? 
They might have bought a new PSU at their local peecee shoppe, which is very convenient. Try finding a 90 watt PSU at those places

I bought a new 460 watt PSU for my A1200 a few years ago for 50 bucks for the same reason: Convenience. Can't be bothered to shop around for low power PSUs. Plus, those PSUs won't have to work very hard, which may be good for their lifespan.