Not to mention that while indisputably the UK currently is a net payer into EU funds, if they don't pay money anymore they will get less benefits out of the deal too.
It would be delusional to assume that they will keep all the benefits while ridding themselves of most of the obligations.
That's just not going to happen, something will have to give.
Another point is that the EU has all the incentives to not give the UK a sweet deal, because that would otherwise encourage other countries to exit as well.