How long does it take you to wrap each vase and what are you securing the ribbons with. That is what should be figured in to your price, not just the cost.
I worked for a very large company that sold ribbon by the bolt or by the yard. There was a 45% mark-up just for by the yard vs by the bolt. I mean, they charged another half percent just to measure and cut. You cannot be shy about charging fairly for your time.
Just for the record, I would decorate the vase and then decide how much I would be willing to sell just the decorated vase. If it adds another $5 for the decorations and the percieved value is there, go with it.