There is a recent post from IamSpartacus on this. I think the sweet spot is $150 or better for a slightly larger Micron drive. Here is the
link from Jan where he recommends the Micron 9200 for $150. I found some at the time from fifakingdom around that price and considered an offer for 6 but was too chicken to act.
I have 2 of the P3600 you mentioned which were connected to U.2. I bought a Supermicro occulink board and Intel backplane for 8 drives based on some threads here. I need another 6 drives + 1 more occulink board to populate the backplane. I think I will go slow and buy 2 more and keep an eye on another oculink card.
The oculink option lets me trade slots for U.2 ports and more flexibility in choice of a new workstation board.