Your estimate is way high. I recently bought a pair of 2'x20' panels on E-bay, the delivered price was $169. That included almost all the fittings and a diverter valve. I bought another $30 or so of 1.5" pvc pipe and fittings so my total cost was about $200 for 4x20.

To heat your pool - probably not - but every BTU you add is a plus. My pool is 20 by 40 IG - and I just look at this as extra heat that the heatpump won't have to produce.

I have not run the heatpump at all since bringing the pool up from 44 degrees to about 74 degrees last month. Today the pool is 83.