Prices are difficult for both.
I've seen singles in good nick go for £50 before, so at £850 they would have to be immaculate.
Same with Ellipsoids, Herminator got a spare one for about £25, I got four in ok shape for £205, but I have seen a set of good ones with worn tyres go for £500 before.
I really think it depends how good yours are, and how good the 108's are, but from what I've seen it's about a £300 upgrade all said and done, unless you are willing to pickup singles/pairs etc, and get a set together, then get them reconditioned.
Dave