Sometimes you talk to a prospect and impress them with your wealth of knowledge.
If you're not extremely careful trying to impress someone can make you seem arrogant. If that happens a future relationship with your prospect is highly unlikely.
What about those prospects who simply want to use your knowledge themselves? Either to start up in competition, to understand what your offer is so they can get it cheaper. Or to simply use it to check what another company they're employing is doing.
What about stopping the FREE consulting?
OK sometimes giving FREE advice is so ingrained it can be difficult I'd be the first to put my hand up for that one. I mean check out the blog!
Still, saying that you expect to be paid for consultations certainly qualifies your prospects. You can bet those that are simply wanting to suck you dry wont want to pay for the privilege.
In addition it'll cut down on fruitless prospecting and reduce time wasting.