Any cheap and good pump for heat pump water heater?
 

So I was looking at the parts for building a heat pump water heater and I noticed that surprisingly, the water pump, at nearly $200, is the single most expensive part in the design. Granted, that was for a good quality Laing D5, but cheaper hot water pumps (without the very necessary variable speed high efficiency motor) aren't very much cheaper.

Why is that the case? It's not particularly demanding mechanical wise. Given what's inside, $50 or so would be a reasonable price. (I'm willing to build my own pump from common plumbing parts and stuff if it would be substantially cheaper and/or have some nice features like a superior control range or bi directional flow.)