Posted By:
WPAAs high as people's wallets will take them 
Personally I think its rather remarkable that people will pay several thousand dollars for them. While I don't agree about the comment regarding the idea that they have all been tampered with....at some point the fact that there is no real way to know for sure the providence of these fine specimens will/should? impact the prices.
This subject has been discussed before...I still think that the last three-four years remind me of the mid 70's-early 80 runup in stamp values...the stock market in the 70's went nowhere and investors poured money into stamps....in the early 80's they took it out and it turned out that there weren't as many "collectors" as folks thought previously to absorb the material and prices for most older high value stamps collapsed.
But there are just as many valid arguments for why the prices will stablize rather than fall....so only time will tell.