it really all depends on the landing page/site and the source of the traffic.
One great site, getting great search results traffic can cause 40% of free players to eventually deposit. Where crappy traffic sources could result in only 4%-6% of free players to deposit. These numbers are true at party poker and pacific poker, over approximately 30,000 free registered players over a couple of years.
Not a helpful stat as far as figuring out the ratio of visitor to a portal that converts, but i think the difference between great and poor traffic sources is the important point. Cheap, real traffic may yield something, but it may take 10,000 impressions to get a depositor. For a search like “Play now party poker”, it may only take 10 impressions to get the depositor, but there may only be 10 impressions, a day.