It's a great question.
We found the same problem over suppliers' pricing models in many technology events; per seat, enterprise wide, tiered etc and often ended up running two stage events with an RFP to understand general pricing models and then an eAuction.
At the eAuction, if we found consistent pricing models across suppliers, great, but more often, we'd present suppliers with our use case and ask them to bid based on what we actually needed. This prevents some of the 'funny money' style pricing options that leave you with Excel and a calculator in hand.
Getting the supplier to price their package is always a better option as it avoids the chance for us to make honest mistakes.
When asking suppliers to bid, we'll often ask for the total invoice price for years 1, 2 and 3 and also include line items such as sign on / switching bonus (but clearly these are upward bidding terms!); the supplier doesn't need to offer anything but ...
Global eSourcing Manager | Reed Elsevier | email@example.com