A: Prices on decorations wouldn’t (at least, they shouldn’t) be dependent on the number of people attending. If I were charging, the factors I’d be considering would be the materials desired (number of arrangements, types of flowers, draping, equipment), time (how much time needed to create arrangements, set up, break down), and location (far vs. close by) — that sort of thing.
I couldn’t really give you a ballpark on what a potential decoration fee would be without those factors. And beware florists and flower shops who give you a flat fee without asking for those particulars — either they will tack on fees for any or every little thing or give you the lowest-quality flowers and decorations available. You get what you pay for!