I would expect it to cost maybe £40 to get it blasted. I would avoid powder coating if you are tempted, especially if it's aluminium. If It's Aluminium then you need to undercoat it with an etch primer, you can get this in rattle cans from halfords.
A not too coarse grit blasted finish is actually best for painting as it gives a better key than shot or bead blasting, which is really a finish in itself, often used in bare castings that won't be painted. Depending on how you do it and the medium used, bead blasting can give almost a mirror polished finish. Anyone who offers the service should be able to advise you, and will know how to go about it depending on the material. Like many things there's more to it than you might think.