You can get cheaper parking by going to the "Resort World" car park which is a quite short walk away from the NEC.
This is from their website.
Parking for two hours is ?3, and 2-12 hours is ?17 but you can validate your ticket on spends of ?20 to park for up to 12 hours for just ?3! Validate parking with a valid receipt at Guest Services or, if closed, at any of our bars or restaurants, or in Cineworld or Hollywood Bowl.
Basically go in and spend ?20 anywhere on food / or anything else and parking is ?3 for 12 hours.