That's not how Forager works. It gives +$1 for each DIFFERENT treasure in the trash. So if there's five coppers in the trash, it's still only gives +$1. If there's one copper, one silver, one gold, one platinum, and one Horn of Plenty, THEN Forager will give +$5. But that's pretty hard to set up.
Forager gets to $1 basically trivially as soon as you trash a copper, it gets to $2 fairly easily if you're building an engine (you probably don't mind trashing that silver you opened with once you have some festivals or whatever), but getting it above that requires either a deliberate commitment (to buy a gold just to trash it) or some extra early-game-use treasures on the board (Loan, Talisman maybe, maybe potion, etc.) or some alternate reason to trash treasures (Remake Gold/Plat to Plat/Col, etc.)