£40+VAT, but that's fully custom - they start with a blank white shirt and print everything into the fabric, so your name, sponsors' logos, etc. are integrated into the design, and there's no decals or stickers to peel or come off in the wash.
Most of the sports shops I've been into (inluding my local JJB) refuse to do lettering on any shirt that they haven't sold you. Because some fabrics are damaged by the heat process, they don't want the liability of ruining someone else's shirt. Either that, or they just want to make more money out of you