DWFII said it very well.

Mark you haven't offended anyone. Wearing inexpensive formal wear v. "Cheap" can be very satisfying if done correctly. Cheap can be anything which isn't up to standard (e.g., expensive designer, yet ugly, Lemon Yellow tail coat at a white tie function where the Queen is present).

If I have a well treated, second hand Geoffrey Tailor doublet that I bought for 50 quid, is it any worse quality than a brand new one that costs £500 (10x)?

Yes, there are certain standards, but there are also some latitude in those standards. Look at some of the variances in doublets in the 19th century.