I cannot provide numbers, but I'd argue that the immediate short term and possibly medium term economic effects wouldn't be that serious a hamper on the British.

That's not to understate the value of the colonies, not at all! For a great while, the Northern colonies were almost the breadbasket of Europe, and would certainly--even if it wouldn't provide a monetary value--be a boon if the British had managed to keep it. Meanwhile the Southern Cash Crops would be a good injection of cash into the economy.. however the major boon the colonies experienced as Europe's various wars relied on colonial resources to continue to a degree would most likely have ended due to various taxes, tariffs, mercantilist trade policies, and so on imposed by the British Government.

Britain would also possibly see a reduction in spending, in part because there would be less massive garrisons of British Troops stationed in the American Colonies, as well as trying to collect taxes from unwilling subjects who were just as likely to destroy your expensively imported goods as they were to buy them.