Probably around £100-120 for a colour plus any shipping, if my previous experience is anything to go by.
So cheaper than fancy wet spray paint (£180ish upwards for a good job), but a little more than acceptable powdercoat (£50-80).
I think it depends a lot on how much prep they need to do on the frame, as I remove all parts, any steel inserts, bottle bosses etc, stickers, and degrease.
I do also remove the old anodising, but that's just because I'm generally welding something on the frame first, I think ths actually prefer to do that stage themselves.
All the best,