There are fewer and fewer auctions -- and, conversely, more and more BINs -- for key Silver books as the market has now become a seller's market. There are still many collectors out there hungry for a chance to own an "entry level" AF15, and the window on affordable, unrestored copies of AF15 underneath $7,500 is closing. So, yes, it's a lot for a .5, but the alternative is buying a 3.0 blue for twice as much, which is unaffordable to many. So entry level buyers are thinking, "buy the .5 [or 1.0/1.5/2.0] or get left behind at the station."
That's my story. Used the same logic when I bought my 2.5 last September for $7,500. Did the research, saw the dramatic climb upward and decided that now was the time to buy before it became (at least for me) illogical to spend so much. Funny, I already want to upgrade.