I don't understand what all the flame is about.
Have him sign where ever he wants, have him sign whatever jersey you want. It's going to be your collectors piece. $200, $30 or $400, who cares? You can't put a price on things that are priceless to you, yes? Don't forget you'll drop a pretty penny on the glass frame, go for UV coated glass to prevent fade.
I went to the Giro last year, I had a much sought after Col d'Agnello cap and a folded up gAzetta del sport which fell out during a bottle/newspaper hand off on the top of the climb... impressive stuff. My friend had an Astana feed bag which he got from soigneur (whom he helped pack Vincenzos feed). I had the best intentions of marrying the two in a nice frame but now the cap is in my office desk and the newspaper is well... I don't know where exactly.
Point is, don't overthink it, spend the money today else tomorrow it won't seem like such a great idea. Black sharpie....sign wherever. Frame. Hang in garage or bike corner. Done.