I’m not seeing a lot of forest greens this season. There’s a ton of peacock and teal (namely from Panache, Claudette, and Fantasie), but none of those are under $50. You may want to try searching eBay for emerald green bras from previous seasons, or buy a bra you like in white and custom dye it yourself.
Edit: Also check with Secrets For Your Sister. I know one of the women that works there loves green bras. So she may have a suggestion.