A friend of mine has such a business and previously used to do the specialist printing for all of Adidas UK sponsored athletes I believe. He also provided sets of kit for me on a couple of occasions. See Linx90
As for cost, it will vary depending on the lettering/artwork you need. With a small number of vests you may similar problems with all suppliers, ie print set-up costs.
Alternatively you could try Ron Hill, Fastrax or Wasp, all of which have a significant prescence in the "running club specials" market