I suppose it's too simplistic to look at Brexit this way?:-

You join a club, you have to be accepted and pay a joining fee. Then, during your membership, you pay your dues and accept the benefits of membership. You decide to leave so stop paying your dues and any benefits of membership end. That's the end of it.

If you want to continue enjoying just some benefits of membership, that's a different story with a different structure of paying any dues. You don't have to consider how much it's gonna cost you to leave before you do, you just stop paying the dues you used to pay and after that, negotiate costs of any benefit from the club or it's members?