Any lead provider that answers this question quoting a percentage is being disingenuous as lead providers don't convert the leads, the lead buyers do!
What this means is that the only figures that lead providers can give you are based on feedback from their customers.
Some lead buyers will convert 10% of their leads, some will convert 15% and some may only convert 5%. Conversion rates vary from lead buyer to lead buyer but the better your approach and the more prepared you are the more leads you will convert. What nobody can predict though is which of your leads will convert.
What I mean by this is that if you buy 100 leads you may be able to roughly predict what percentage overall will convert but it is impossible to predict whether these will be the first 10 leads you receive, the last 10 or somewhere in the middle.
Although this point may seem obvious to some it is often ignored. When you are about to start your first lead generation campaign or in fact any, you should consider the above principle at all times. When a lead buyer asks how much they should spend on buying leads the lead provider will often reply something like "you need to make sure you buy a decent amount of leads over a period of a few weeks".
This reply is not meant to sound vague but it is meant to help the lead buyer. If you only spend £100 for example and receive 10 leads, the performance of those leads is just pot luck. They could all convert or none of them may convert.
The point is that as a lead buyer you need to commit a budget big enough to ensure that you can make a useful analysis of your lead buying campaign. This can only be achieved if you commit to a decent volume of leads which could be anything from 30 to 100 depending on the lead product.