Hi,
you might want to check out shapeways.com . It's a company that does 3d printing (SLS) for "normal" people. They are based in the Netherlands. The prize for "strong + flexible plastic" (PA 2200) is $1,40 per cm³ (white).
I did a gearbox for my Academy GV2 mid-motor conversion, because I wanted an option to run 3 or 4 gears. Here is what my cad drawing and the final printout looked like:
My gear box has a total volume of about 20cm³, shipping was $9,50 so you can do the math (~£25). I don't think you can get it cheaper than that.
You might want to optimize material thickness on your model though to reduce costs. In addition to that, they have a handling fee for each part, so what I did for my three parts (the small one in the picture has nothing to do with the gear box) I connected them with a small link, so the whole assembly counts as one part.
Accuracy was fine, even the gear mesh, holes and fittings tend to be on the smaller side, which I prefer anyways.
Andi