I believe (after having spent some time studying this issue - but not enough time to have the right to describe myself as an expert) that "paying for market access" is kind of an "European anomaly", that the UK and other nations, like Norway and Switzerland (which are members of the European Economic Area) should fight with more energy. It is actually (in my opinion) a kind of financial extortion that everybody seems to find normal, but which is actually done for partly idealistic and social reasons. The (perhaps dubious) idea which governs this "payment for access" is that free and frictionless trade is hard for weak economies, therefore these economies need financial assistance (reference:
http://capreform.eu/the-uk-must-pay-for-access-to-the-single-market/ , already cited). So, in a simplified way, if the UK, after it has left the Union, wants to join the single market, it will have to pay an enormous amount of money for Romania, Bulgaria, Greece and so on, because free trade is allegedly so hard on them.
(You have to realize that free trade intrinsically costs nothing or almost nothing. Of course the UK may have to follow some European rules, which can be downloaded on the relevant European agency, but the cost of this is almost zero.)
Now, it seems to me that one can answer to that, that free trade (not having to fill out customs forms, not having to pay tariffs) is actually good for everone, including in Romania and so on. But it seems that almost nobody is doing this, everybody is fascinated by the "European snake". I am not convinced that medium-income countries like Romania really need financial assistance, aid should be given to those who really need it, and there should a budget to end climate change, to build advanced nuclear reactors.