As others mentioned... You cant really quote on price because you're not doing it in house. You could quote the customer $20, then the sign shop could tell you "No, thats 10x more work than normal, we'll do it for $50", then you're SOL.
Get a sample file of what you plan on doing, send it to a few sign shops in your neighborhood and ask them how much they would charge, then double, or triple it for the customer.
You could ask how much they'd charge just to cut... But odds are they wont, unless they are really, really slow, and you have a good relationship with them.
You can buy a decent starting cutter for a few grand. Take the plunge and buy one... You may not get a lot of jobs with it right now, but start marketing it to people, and you never know what will happen. You'll also be able to sell just plain cut vinyl decals. A plotter is one of the main pieces every shop should have, it opens the doors to a lot of business.