Benchmarking Project Mmgt Effort

I have recently been reviewing a tender for developing a corporate email system. I was a little concerned over the effort that was being attributed to the Project Management task. As a result of my concern I did a quick look at similar project in my department and found that PM effort levels varied from 15% upto 30% of the budgeted effort. In light of this large variation of effort I want to get more data so I could refine the metric I would use. With this info I would then be able to provide evidence to negotiate the tender figure up or potentially down.

I would be grateful if anyone can provide me with a rough estimate from project that they have worked on or alternatively point me in the direction of a web site which contains such information.
TY
FraserH