Article 50 gives two years to extract. It is actually both for renegotiation and to prepare for leaving.
Cameron has, rather foolishly, refused a further referendum to accept or reject a renegotiation in light of an article 50 so two years it is then out.
Norway pays a couple of billion per year and Switzerland far less as contributions for access to the single market. However they both enact most ( about 80%) of laws and directives and impose them upon all of their companies. Saying that their bulk exports to the rest of the world are very limited so it makes some sense for them.
There is no explicit need for this, only the 6-8% of businesses which actually trade with the EU should technically have to accept all the European standards in this country should we leave. Hence companies which trade with India would have to abide by the Indian's standards, not the EU ones.
I can see some problems here, but advantageous ones in the main.
As for how much would we have to pay, that will be part of the two years of negotiation. Cameron hasn't exactly given himself a strong hand to work with here with his insistence that the four horsemen of the apocalypse will strike hence it might be on the high side.
Certainly a lot less than we currently pay and technically there is no particular reason why we should pay anything barring the EU's intransigence and the incompetence of our politicians.
They export £30-40 billions worth more than we export to them so tarifs would actually be to our advantage on paper. More so when they are no longer allowed to plunder our fishing free of charge. Most of the EU fishing fleets are from nations that are in some distress already, particularly the Spanish so the loss of this would be a read headache and drain on the EU.
Interestingly a lot of the problems off the horn of Africa are actually due to EU fishing fleets being given rights to plunder there, which resulted in no fish for the locals who resorted to piracy. Hence it is fairly clear that the appetite for fish is strong and one that we would provide, at prices set by ourselves.
Whilst the remain side with all of their tame economists claim figures such as £9 billion gain the EU itself is figuring on a £30 odd billion loss to it's accounts. That isn't necessarily a net gain on our side, more likely somewhere in the middle however few seem to realise that VAT is an EU tax with a proportion going straight to it's coffers. It was a requirement for membership in the early 70s and alien to us before that.
VAT on fuels in particular is a growth destroyer and I'd expect it's removal alone would provide sufficient impetus to the economy to negate any fees or danegeld demanded.
The EU also benefits from the quality and efficiency of our agriculture. One rarely mentioned side effect of Brexit is the likely monopolization of the common agricultural policy farmers ( particularly the French ones) who frankly could bankrupt and ruin a superpower given their demands.
Who nowadays could conceive of a more eco friendly and efficient system than our dairies of old. Recycled glass bottles, electric vehicles and all local and freshly delivered to your door. Twas the EU farmers wot killed them, oh and our own politicians embarrassed by the milk lake, butter mountain etc.
My thought, though I am no expert, is that given a year they'd both be begging and paying us to remain in the free trade zone though I have no faith in our politicians to deliver such a deal in our interests.
So in summary our position would be strong, very strong, but our politicians weak.
Twas ever thus.