I have purchased many bras on e-bay (the brands I like are in the $150 range in Canada) but you need to take care when selecting sellers. Many sell seconds with a note that there are no visible flaws which is true. The flaw is that they fall outside the acceptable size parameters. If you choose sellers that sell only perfects, you can get a very good quality bra for a fraction of the cost. I also buy my sports bras online. I think they are about $100 in the store here but they only carry the most popular sizes and I don't fit into that range.
I've heard M&S are very good bras but for larger cup sizes, you can't beat Fantasie, Freya, Fauve. This is the same company that used to make Rigby & Peller.