Amazing collection, bt. What is the rarest released PCE/TG game, anyway? Dead of the Brain maybe? Dynastic Hero US?
All depends what you mean by released.
Commercially released would probably have to be "Akiyamajin no Suugaku Misuteri - Hihou Indo no Honoo wo Sisyu Seyo" -- This wasa Mook release, and reportedly goes for about 150K Yen when it is available.
As for the rarest non-commercial released games, probably the 2 gold HuCards (Power League All Stars and Youkai Douchuuki). When seen, these sell for 200K to 250K Yen. I am not sure how or why these were released, but I know the numbers were few.
Just my opinion.
-bt