The Gleeful Mask Rucksack randomly gives out 2-3 masks including that one. The bag is priced in user shops at 6mil right now. Sorry I couldn't help more.

I didn't think of searching for the sack. The mask itself has an average buy price today of about six and a half million. I figured it would be more expensive and sought after than the sack itself.
I'm not sure how in demand that particular mask is but I think it's important to know that it can only be obtained through chance. Hopefully you get a better answer on this thread and good luck selling.

Whenever an item from a chance bag is being priced, I usually tend to estimate a little higher than the bag's price (If there are none in shops/trades). So if it's 6m in shops right now, I'd say anywhere from 6.5-8.5m is good. But like said, it gives 2-3, so you'll have to be the judge of what price in that range is good :o