According to Brian Olsen (I think it was him), who used to work for ICE many, many years ago, there was a formula, and it it did get changed, multiple times (which explains why similar TPs had such varying costs). Brian then released the "current" formula shortly before he left ICE and a wonderful lady named Shiela from the RM mailing list created a spreadsheet using those released rules.
That spreadsheet can be found at http://www.ironcrown.com/ICEforums/index.php?action=tpmod;dl=item456
That spreadsheet was used to create the Master TP Cost lists that are also found in the Vault. The Master List is the only place, I think, where you will find the costs for all TPs made all using the same formula.