If you could supply a little more information, I suspect others will be more helpful. But I am assuming that this free membership website gives your client information from other vendors or other sites? Is that what you mean be "trade off of information"?
One suggestion is to offer your vendors some type of referal or affiliation program. When free subscribers of your site click their link for their information - you get some type of revenue for the referal.
For example - take a look at the Amazon.com referal program. If you lead your subscribers to their site to purchase an item (book, DVD, CD are typical informational items) - you receive referal revenu as an affiliated partner.
There are other ideas as well. Give me a call if you want to discuss further.
See my earlier posting on 3 tiered memberships and backend purchases of goods and services. If you don't think about your backend you can work your backend off with little or no profit to show.
Michael T. Irvin