My experience has been 5-10%. I've tried direct mail for several different ventures over the years and have not ever been all that impressed with the results.

I'm a previous founder of 3 SEO companies but I'm not biased in saying that inbound lead generation via the web is almost impossible to beat as far as customer acquisition costs are concerned compared to traditional marketing like direct mail.

Those stats are fairly well established. Of course, there may be a particular demographic you are targeting that have tested positive for direct mail affinity and in that case you will see better than average results.