At first I thought the domain in question was "IRS.com"... Which, ironically, is underdeveloped as a tax resource directory.
Frankly, I think IRS.com would be worth more than tax.com, simply for the controversial value of it.
In any case, tax.com stands to be a big one, definitely at least a $5 million name.
A good purchaser would be salesforce.com, in my opinion. They've touched on virtually every other aspect of enterprise, and it fits right in with their naming patterns (e.g. Site.com, desk.com).