As of January 2017: (Prices might have changed)
£175 per bolster (Lower or Upper)
£285 for both
That's per seat, apparently it's the same price no matter how bad the bolsters are (within reason I assume) so it's probably just worth leaving it for a while and getting it done when it really starts to annoy you.
I've not a chance of my girlfriend contributing either mate so I'll have to start saving too!
I think I read somewhere direct from Renault a new complete base is ~£170, so I might give them a ring and inquire.