I am new to this site and so here goes.
I have been collecting English Bible translations since 1968 and have seen a great increase in what you must pay for them. Yes, the dollar value has gone done but the difference is to large to be credited to that. However, I was a seller and a buyer on e-Bay. I believe that e-Bay is the cause for the increase in the price of Bibles. I agree with Ambrose completely for most people do NOT know what the real value of Bibles should be. They see a Bible 100 yrs old and say "wow, I have got to have that" and will bid up the price until they out bid others who don't know the true value either. Thus, inflated prices. The same can be said for the sellers.
Normally, Bibles are not considered old unless from the early 1700s and back. Normally speaking, there are a few translations done by individuals from the 1800s that have a high value. Most, but not all, Bibles with errors have a preminum value. However, only those from about 1830 back to 1530. (A list of these are in my book.)
Because sellers on e-Bay, no matter the reason, get high prices for their Bibles, professional book sellers are now pricing their Bibles based on e-Bay prices.
This is also true for most books today. An example would be my book. It's still in print after 17 years and sells NEW for $295. I just went to ABE.com and two were listed used. One priced at over $400 and the other for over $700. Both are forsale by professional book dealers. Now, who will ever purchased from them when the NEW is selling on amazon.com, B&N, and other places much, much lower than their price for used copies.
I have found that if you don't buy at the inflated price, you will never be able to buy what you want or and need. That is exactly why I have not been able to purchase any Bible translations in the last three years.
All of this makes it rather hard for me when I do an appraisel for a Bible.