Please allow me to make a small correction. If you decide to register for the Nationals at the convention, you must first join IPMS/USA if you aren't already an active member. This must be done separately at the IPMS/USA booth, which is usually right next to the registration booth. Unfortunately, Society membership cannot be combined with registration, as it would violate IRS regulations.
Based on each convention's policy, the walk-in fee might be deducted from the convention registration fee, not the membership fee. There is a provision in the convention registration software for doing this.
Also, the venue for a Nationals is considerably more than a couple of grand a day. The two largest expense items for a National convention are venue and trophies, respectively, both of which are in the low five-figure range.
"You can't have everything--where would you put it?"