Companies generally acquire each other, speaking very loosely here, for X times Annual Revenue. If Wizards was purchased for $350,000,000, and Hasbro _really_ wanted it maybe they paid 10 times annual revenue. That would have made Wizards worth $35,000,000 in 1999. If they paid only twice annual revenue, Wizards would have been worth $175,000,000 in 1999. Two times annual revenue would have been a deal for Hasbro, however, and it seems that the Wotc stockholders thought they were the ones getting a deal. So (discounting the high end as well), Hasbro probably paid between 3 times and 8 times annual revenue, setting Wizards worth at between approximately $116,000,000 (3x approx) and $44,000,000 (8x approx) in 1999 in terms of annual revenue.
Allowing that Wizards big success stories were Pokemon, Magic and D&D (all iterations), in approximately that order, in 1999 and, generously allowing that each made about the same amount of money, and further using the $116,000,000 dollar figure (3x annual revenue), D&D would have been worth $39 million dollars (approx.).
Allowing for growth with 3E to double D&D's value between now and then, it would presently be worth $80 million dollars. This figure is likely high, however, as the 3e bubble has come back closer to the ground. It is also high if any more that 3x annual revenue was paid.
Then, to Charles point that D&D is worth more than people imagine, I believe this likely true but it is a highly relative claim. Completely unscientifically, I'd place D&D (all iterations)annual revenue at around $55 to $60 million dollars (and I think I may be high by $10 million).
Now, if you are a buyer, you are going to pay X times annual revenue, as well. D&D could then be worth $60 million dollars in annual revenue but be worth several times this at least in terms of any purchase price. I'd venture a sale price of $175,000,000 would get the job done on annual sales of $49 million dollars.
Since I'm no good with math, and am working from publically available numbers, take all this with a grain of salt.

Suffice to say, Charles is right in one sense but less so in terms of absolute annual revenue, if these numbers are anywhere close to the bone.
