I just replaced two knackered rear calipers on my Fiesta ST - these are the same as on the ST170 that you've had prices for, only the carrier is different.

The 'genuine' Ford parts were indeed silly money, I wasn't quoted quite as much as your £440 each, but the bill still but came to £700 and a bit for a pair, that was on an exchange basis as well.

I opted for the slightly more palatable option of Pagid calipers, saved me a bit over buying genuine Ford parts, but still steep . Had I had time to take the car off the road for a while, rather than needing it for work that week, I would have taken the calipers off and seen if BiggRed could refurbish them or something.

Alas anything ST/RS specific and going for official Ford parts and the garage will have your pants down, and the other parts manufacturers (Pagid, Lucas, ATE or whoever who also sell compatible parts), whilst cheaper than Ford, will still be expensive. These rear calipers are significantly more expensive than the fronts!