Whatever the gp-value is times .35-.40. So a 2b hat would be worth $700-800.
Few legitimate people would offer more than .40/m, please be cautious of trading with anyone who offers more than .40/m for your partyhat.
Since you're new to sythe, I'm happy to act as middleman for the sale of your hat if you want (Feel free to infract this post for advertising mm services, I'm just trying to keep people scam-free)