Ask NH: How much is good marketing worth?

A couple of friends and I have a start-up business. We have all the technical know-how and are confident of making a good product. A contact of mine is interested in the project and wants to participate. She claims to have a good network, incl. Techcrunch et cetera, and would use it to promote us. She also made some money from an earlier successful start-up, whereas this is the first start-up for everyone on the current team.

How much is she worth in shares of the company?