You would be surprised how big those dandelion bunches are, but I get what your saying.
Not trying to be argumentative, but it does not seem like you get what I'm saying. I'm not exaggerating when I say that they can eat 20 bunches a day. So by your price that would be $30 a day, or $900 a month. I don't find that to be practical for a large tortoise keeper over the course of an entire winter.
Handouts from the grocery store would be cool though and that is a good idea. Some stores will let you have the trimmed cuttings from their leafy greens and if you mix that with some rehydrated chopped grass hay, it would get you through winter relatively cheaply. I'll bet if you talked to a few of them in your area, you could get quite a bit of stuff for free.