It's sometimes difficult to figure out whether a quote is reasonable or not, and it often depends on what your expectations are as to the quality of the result.
If you expect to get a perfect, invisible match that is going to get rid of the bubbling completely and permanently then the wings are going to have to come off, and the wings are going to have to be properly cleaned and stripped back. Better to have all 4 wings done to avoid a visual mismatch between front and rear, so it's bound to cost a bit
On the other hand, you could probably find someone would would rub the visible corrosion back in situ, prime and paint it with a rattle can from a supplier that will provide the original colour code. This would take a day or two, so you could probably get this done for a few hundred pounds. Paint fades, so it won't be a perfect match, and local repairs like this are never permanent so you could expect the corrosion to return in a few years, but it may meet your needs. Given the increasing enthusiasm for people to publicly criticise or even litigate if they're unhappy with the result of work, it would be a risky job for any tradesperson to take on, but you might be able to find a Chipsaway or similar operative to give it a go. When I tried this route to deal with paint chips on my front wings, I was met with flat refusals to take the work on, on the basis they couldn't guarantee a perfect result.
Alternatively you could accept that this "very minor sign of corrosion" is a nice bit of patina that shows your car is ageing nicely, do what you can with protective waxes to slow the advance as much as you can, and just live with it happily by standing a bit further back from the car when you admire it. That's what I do with my car.
