Don't feel so bad about the experience. At least your bands writes better tunes than the band you opened for.
It sucks you were treated that way but I'm not surprised to hear that though. I've seen some club/bar promoters treat local bands like shit in the past. I'm not surprised this happened on the big stage. The price mark up for your merch sucks to. You can't expect people to pay $40 for a shirt of a band just starting out unless they are really blown away by you. Hopefully, you will get a chance to open up for a well known prog/power metal act soon that way you can at least play music for a crowd that would appreciate it.
3/20 - Dream Theater