I don't have an opinion because I'm kinda in the same boat. I've just sold two CGC slabs this past week and both buyers contacted me saying how undergraded the book was and that they were going to crack it and resub it.
with that said, I may need to seek out cheaper and time effecient alternatives especially if this mixed bag continues. The big hold up with me purchasing PGX books were restoration detection, but in light of certain events-my reservations have been abated.
In terms of PGX prices, I have no clue. A rule of thumb may be to expect 80% of the equivalent CGC GPA price. For example: I snagged a HOS #122 PGX 9.4 for about $40; normally a CGC price would neet anywhere to $65 to $100 depending on demand. If you have profit room from there, then do it. Warning: I've just used the PIDOMA method; it's a term I use for my project management analysis. A no prize to whomever first correctly guesses the acronymn.
In the final analysis--perform a JIT analysis of the market when you sell and guage the market. So far, you've done well and I don't see that trend changing since you already have a working formula that just needs small changes.