According to one source I found, you could buy a silver watch (medium quality probably) in mid-17th century for 5 Guineas, that is £5 5s.
If on the other hand you already had a watch, and was going to duel, you could instead buy yourself a siver hilted sword.
Or if you were so disposed, again according to the same source,
you could spend the money on a night out, including supper, a bath and a fashionable courtesan.
I'm not sure about the inflation during late 1700s, but it looks like watches may well have been affordable for the middle classes in Jane Austen's time.
What was it Jane's pianoforte cost? Can't remember now.
Leif Gn