With this one, one challene was how to hollow it out without specialized hollowing equipment. The other challenge was sanding the inside - sticking your fingers in through a 3" opening with a sharp edge on a lathe spinning at 1200 RPM is a little troublesome - the sandpaper would catch and my fingers would get bounced around inside the rim like a marble in a bowl that you're shaking - ouch. They make specialized tools for both jobs, but unless someone orders enough of these to cover the cost, I'm doing it the old hard way - hence the extra charge for enclosed forms.
Enclosed bowls are the price of the wood plus $1.50 per square inch of profile. This bowl would be about $19.75 plus shipping.