The way I see it, there are three variants on Chapel in Dominion. Chapel costs $2, Forge costs $7, and Count costs $5.
Chapel is the version that is extremely underpriced, so that its strength can at least be equal-opportunity.
Forge is the version that is slightly overpriced to make sure nobody gets it for the first few shuffles. Forge's ability has a slight edge over Chapel so it doesn't feel like you're being ripped off, but I think it would be balanced if it were identical to Chapel.
Count is priced in the mid-range, where players might get access to it in first, second, or third shuffle depending on their luck. This makes it the hardest to balance. So as not to make the game unfair, Count's trashing ability is slightly weaker, often thinning by only three cards, and then ending your turn. But this trashing ability by itself might not be interesting enough to compete with other $5s, so Count can do other stuff too.