I believe the back comes off in one piece most of the innards are attached to the front now. I don't know for sure though. That may be why it's cheaper.
£35 to replace the back of an iPhone 5 is too cheap! All of the innards are connected to the rear housing, replacing that is a very difficult and time consuming job. Genuine Apple rear housings go for around £70-80 on eBay to give you an idea of the cost, whereas the glass sections are £5-10.