Andy, we've talked before about your place, in relation to something a little different, but the same consideration applies. If you have a tree in the area, or the soil is subject to heave, laying a concrete slab may not be sensible. And, as you have found out, not particularly cheap. If it were my shed I'd do something with a bit of give in it. Dig six holes, drop in some concrete by way of a foundation, build nine inch square brick piers off that, with a BAT strap running down each one, bolt treated timber floor beans to the protruding end of the BAT straps leaving a bit of air flow under the beams, and joist between the beans with galvanised hangers. Cross bonded ply over the top of that screwed well down. And, if you want an anchorage point for locking bikes, sink a couple of bags of readymix in to a hole with a stainless steel chain or shackle cast in.
(with apologies to Brains, whose method has stood the test of time...)