Current website quotes £12 per tonne or part thereof. Plus VAT that's £28.20 for you average four-place spamcan, which are generally just over a tonne. As for the facilities that you'd expect for such a lumpy fee, well, freecall Sheffield A/G Radio...
At Biggin, considered an expensive GA field even to the South East's standards, it's radar vectors to the ILS straight off the airways for £24.56 all in.
I think Sheffield need a reality check with their prices to be honest. It's a real shame because apart from that it's a great little airport with friendly people let down by a crazy charging policy. Sorry, it's Gamston every time for me (as it was before EGSY accepted singles).