The only reason to pay the 350-400 you mentioned was for the type of work and quality you put into your set. If you’re making a standard set of cornhole boards, you can use 2x4’s for the frame and legs and 1/2" or 3/4" ply for the decks. By the time you have hardware invested, assembly time and/or paint, you can easily market your boards anywhere from $160-250. The prices you also listed were pretty reasonable for material costs as well.
I have been building a set currently, pictures to come later, and I plan on selling those for around $300-350. This decision is because of the extra detail I’m putting into the boards.
Any other boards I plan to make, depending on details, I will sell for between that $160-250 range. Also note that you should look into your market for pricing in your area. Some people are willing to pay for a well crafted set of boards but others may not want to dish out that much. A lot of it has to do with what you’re able to create and what options you’re willing to offer moving forward.