Bought a pair of Pagid rear discs from Halfords a couple of weeks ago and they came to £69.74 for the pair including VAT which I thought was pretty good. And it's cheaper than ordering them online off the Halfords website (£75.76)
Two piece discs usually have the bell in a different material, both for weight and thermal properties.
The way they are connected is intended to minimise the heat transfer to the bell and wheel bearing (there are aftermarket options that are better than others of course), and also allow for differential thermal expansion of the disc (aka ‘floating’) via the rods that connect the two pieces.
Based on those prices I think I got a pretty good deal at RBM last week!
Ross doesn't split his invoices between parts and labour but I got my new OE front discs, pads and wear sensor for just under £720 including fitting and VAT.
They are sided if you care to take a look at them, as they cool by rotation. Surprisingly often you see kids put on the wrong sided discs! Like children putting their shoes on wrong...