When Wallace died, his books were holding steady, at best, as a collectible. His most recent novel was still Infinite Jest, which was 12 years old. Novels are what really drive collectors, much more than story collections or, worse, essay collections.
The top price for Infinite Jest when he died was $185 for a mint signed copy from Laing Books. Other signed copies were available for less than $100. Now the cheapest signed copy is $895, for a remaindered copy in a crappy jacket. The top price, from modernrare.com, is $2,500.
The Broom of the System, his first book, has always been tough in hardcover, because it was done in a simultaneous paperback. Supposedly there were only 1,300 hardcovers. The top prices for signed copies before he died was $1,500 from Serendipity, a copy from Larry Moskowitz that he had been trying to sell for approximately forever, and a $1,500 copy from Royal. Now Royal has a briefly inscribed copy for $3,500, while Vicarious has a signed proof for $7,500, about seven times the going rate last month.
Even the Girl With Curious Hair, his most common book, has an asking price of $700 for a signed copy.
People will be asking high prices for signed Wallace books into the indefinite future. A signed or, better, inscribed copy of Infinite Jest strikes me as a reasonable purchase, although not at $2,500. It's his key book, and will be celebrated after the journalism has faded.
ABEbooks, the used and rare site, reports that Wallace was their bestselling author in September.