Full disclosure - I am on the committee for the Greater Triangle Friends of NRA banquet. Each committee runs their banquet a little differently and I have not been to the Hickory event, but I can try to answer your questions through the filter of the event I participate in.
We don't have a dress code but most folks seem to fall in the casual/business casual category. You'll be there for a few hours so most importantly, be comfortable.
As for money, it's a fund raiser, so bring what you feel comfortable donating to a good cause, IMO, while having a chance to win guns and gun stuff. Our event has a number of different ways to win prizes (mostly guns but other related merchandise too) like raffles, games and auctions. Last year we had an African Safari up for auction that somebody got a really good deal on. Again, each event is different.
No, you absolutely do not need to be an NRA member. Everyone is welcome. I don't know if it's against CoC to post a link to the event I am a part of so I won't, but here is the main Friends of NRA website.
https://www.friendsofnra.org
Hope this helps a little. Have a great time!
Nate