What to Charge Client BPMU

Hi All! I have been customizing WP themes for a couple of years and this is my first venture into WPMU and Buddypress. My client wants a BPMU site with ecommerce.

Any ideas what to charge for BPMU with ecommerce? Client wants to create an auction site.

And – what kind of maintenance would a site of this complexity need? I am thinking 20-40 hours per week to maintain it? What could the client expect to pay for on-going site maintenance?

Input and info very much appreciated!!!