The reason for the expensive plug is that PR buys original plugs retail from T/C and simply machines the 209 cavity out to the correct size. If you had access to machine tools, you could easily do it yourself. But you'd probably ruin a few hundred dollars' worth of plugs getting it right.
T/C does not manufacture thousands of extra plugs. Cecil stated back when the .25 ACP plug first came out for the Encore/Omega that he could not buy enough plugs to keep up with demand without going to the exchange program. Now the Prohunter plug is a new design, not shared between two platforms. T/C likely has even fewer of them on hand for Cecil to buy up. So if he modifies YOUR plug, you just pay the labor, machining, Cecil for his designing the modification, and a profit margin. If you don't want to modify your plug, he WILL sell you one of the small number of new ones he probably can get but you will pay for all of the above PLUS the cost of the new plug.
C'mon Oldsnow, is that so hard to figure out?
And yes, PR bullets deform over 2000fps and lose accuracy...1800fps in some guns. PR will say not but I have tested most of them and agree with you. Between that and the cost I shoot the 200gr Shockwave now.