It depends. 0% to some really good deals.
You also up your minimum order to $40.
You give up promo codes (e.g. $30 of $300 order). Midway tries to keep you from having a dealer and non-dealer account simultaneously. Once you have an FFL account, your address is flagged so you can't have another account at that address.
Years ago when I first got the dealer discount for a C&R it was quite substantial. Now it is next to nothing, I get better prices from Graf's and rarely use Midway anymore.