I bought my phone off eBay virtually brand new, unlocked and came with a new PDair case and official Samsung extended battery all for £351.57. I am on a £15 rolling monthly contract with Tmobile giving me 350 mins, 300 texts & 500mb data and although I am happy with them, I am switching to '3' network this weekend because they offer 300 mins, 3000 texts & "all you can eat" data for £15 per month.
A friend at work has already made the switch and he is receiving upto 6,000mbps download & 3,000mbps upload speeds.
So my advise would be to consider buying your Gnex outright and forget about being tied into a lengthy contract.