I pick up PSA 9s exs as a main part of my collection and $100-$300 is a great price range for picking up many of them today though some can fetch higher for sure. I can attest to @wisewailmer 's point of 2007 sets seemingly having more printed than 2005 counterparts. Power Keepers is one of my favorite sets and growing up I would find it around in products into the DP era. At the end of the day these were still set cards though so there comes a point where being scarce sets in this case means still being printed in mass just less than other eras.
Keep an eye on pop reports and sold prices when looking for cards is what I would say so you can get a feel on the more popular cards and the cards that don’t pop up as much. While I have been collecting Dragon Frontiers 9s doing that has allowed me to understand how popularity, scarcity of cards, and price interact to an extent.
Here is a thread I love discussing print sizes of the era so you can get a feel for that too 