After Hewlett-Packcard cut the price of its fading TouchPad tablet to as low as $99.99, the gadget has been selling like wildfire from retailer shelves.

While HP hasn't released an official number of units sold yet, perhaps we can make an educated guess.

Both HP and retailer Best Buy Web sites reported that they had sold out of TouchPads.

Due to the significant price reduction, we experienced overwhelming demand for the product and are temporarily out of inventory, HP said in a statement on its Web site on Monday.

Best Buy reportedly had 270,000 units in its inventory.

Both HP and Best Buy also assured early TouchPad buyers that the companies will refund the price difference between what they previously paid (as high as $499) and the current firesale price.

Amazon.com also reported that the Touchpad became its top-selling electronic gadget over the weekend.

Other retailers also reportedly sold out of the TouchPads including Barnes & Noble, Fry's, J&R, MicroCenter, Office Max, Radio Shack, Sam's Club and Target.

Wal-Mart also sold out of Touchpads online.

Moreover, the discounted price has now reached the UK and Europe.

HP’s Social Media Manager, Bryna Corcoran, posted on Twitter that its warehouse would be sending out another consignment of TouchPads, although she didn’t provide any hard numbers.

No more [TouchPads are] being made, she tweeted Sunday night, but [we] have inventory coming from ones already manufactured.

Thus, using the 270,000 inventory of Best Buy as a base figure, and adding all the TouchPads that have been sold by various retailers (both online and onsite) we can make a conservative guess that at least 500,000 units have already been sold, perhaps many more.

As more data comes in, that number is likely to rise.