'V' groove routing at cheap PCB manufacturers
Usually these board houses are really charging you by the FR4 substrate sheet. What I usually do is wait to have a few designs ready to go, and then "panelize" them. If you order a whole sheet it's usually cheaper, because you save them the trouble of having to panelize your design with others. You can do it yourself, and supply them Gerbers for the complete sheet, using up as much of the space as you can. In those cases, if you include the V groove on your panel, they should do it as you asked. If they have to panelize your design with other customers, there is a good chance they will refuse to do it, because a V groove affects both sides of the line.