As negotiation is still ongoing I cannot say with any degree of certainty what the possibility is of damage to the UK economy following Brexit. Nor can I put a value on the damage (if any).
Also I cannot put a hard cash value on the future benefits that I believe we will see once we are able to trade with the rest of the world on terms which we negotiate, rather than terms negotiated by the EU with a view to protecting their internal suppliers.
I can, however, state that it is my belief (shared by others) that the UK will be quite capable of dealing with world markets once we have escaped from the EU. We will also be responsible for our own laws.

Persons who are supposed to have a much greater knowledge of economics than I have have given their predictions. Those included predictions for the immediate post-brexit-vote period of massive increased unemployment, a £60bn hole in the UK's budget, and an immediate punitive budget to get us out of trouble.
Are you surprised that, having decided to ignore 'Project Fear' the first time, I am happy to see Brexit going ahead, and see nothing to fear from our current negotiations.