I used to keep an eye out on e-bay and they make a fairly decent price. Upwards of $1000 I think, depending whaty it's signed on. I don't remember how it came about but somehow I had a phone call from soemone from the AFL who is involved with memorabilia kind of stuff and he told me soem things to look out for because there are fakes.
Sorry I can't remember the details, but one thing was what it was signed with, ie black texta, pen or pencil. The RFC had some posters with Jack's sig, amongst others, so it might be worth gving them a call, and also ringing the AFL and ask to speak to someone for advise.
A cheaper option would be to buy Otto's signed playing jumper with COA from me instead, haha.