I have participated in marketplaces that charge for access (subscription fee) and ones that do per transaction charges. I have found that I prefer the per transaction charges as I am essentially paying a commission for the lead / opportunity, which is much more palatable. I have worked with platforms that charge a fee for leads, which is more akin to a subscription fee. The problem is that you are asking providers to pay for the hope of making money where as on the per transaction you are asking providers to pay for actual revenue.

Elance and Upwork have a hybrid model that also seems to work well. You can bid on so many jobs for free and pay a percentage of revenue. They have higher levels where you buy more credits to bid. On Elance I started on the free level and had success so now I subscribe. This is an interesting form of the freemium model.

Hope this gives you some insight. Let me know if I can be of further help.