You can buy a replacement (steel) rear triangle from Brompton, it is an ordinary spare part and way way cheaper than 750$. Did you ask your Brompton dealer about getting a spare rear frame or just look on ebay? Depending from the country you are in frame parts may not be sold separately to end customers but only for intallation at a Brompton dealership. With the rear triangle this is a good idea anyway as you have to exchange the bushings and this is a bit of a delicate, but not very expensive task. If you have not done it before I'd highly recommend delegating this operation to a workshop anyway.