People already do this, so its not a new idea. It especially works for trades who don't understand they are paying over the odds long term but need the leads and think this is better than adwords because they basically haven't got a clue. However you can end up with a limited lifecycle business.
To explain:
When they eventually figure out the domain name and the email address isn't theirs (because it can be sold on to another person if they decide not to continue paying for it as I assume you can only use a generic domain name not a branded one or their own current one if they have one) they will leave eventually.
Its fairly easier to get a website to page one for local keywords, so what they would pay you monthly over say 1-2 years, they could pay me the same money and have a mobile friendly professional custom design website of their very own, hosted, and the seo to get them to page one, for a lot less than I see people charging for this option (typically £50 a month), hence over 2 years it starts to work out expensive. I'm not self promoting either - but this is an example of what you are up against, and once they have made money from their leads, their natural next step is to have their own site generating their leads for a whole lot less. Hence they go to a web designer with their wodge of cash they may have made form a year's leads of you.
After a year when they realise the amount spent over a year and they still don't own it and could lose that to a competitor, that's when you lose them. I have had these people come to me.
Ideally, you want a business model that retains your customers long term, so what I am saying is look at the cost long terms and view this as keeping them long term and building the customer base, and not charging a fee that after 1 year, makes them realise its not worth doing as all you are doing is ending up having to keep finding new customers. Obviously they wont all do this, but I am calculating by experience of knowing the nature of it all, many will, and that's not a wise business model if you are serious about it.