Until the Openreach product is used by someone other than BT Wholesale they're the best to look at and according to BT Wholesale the uplift they're charging ISPs over the 100/15 FTTP product is 13GBP/month, not including VAT.
Add the extra strain on interconnects and transit and you soon end up with a product significantly more expensive. The vast majority won't bother; the uptake for Virgin's 50Mb product when it was 52GBP/month, and frankly a 330/20 product will have to be shaped senseless to come in that cheap, was tiny.
I ask because I want as decent an upstream as possible.
Now 100/20 or even 110/30, that'd make for a very appealing product to me!
Openreach are quite intentionally keeping the FTTP products highly asymmetrical.
I've heard a couple of price points from BT Infinity for the 330/20 product which are relatively cheap but come with restrictions over and above the existing ones.