I now have all six volumes and, yes, they have gone up. At lot.
I was lucky and paid about $300 USD a piece for all but one. That one I had to pay $900 USD for and that was just to complete the collection. That included shipping and handling and in some cases a "finder's fee" and so I paid about $2400 for all six.
It took me over three years to get them all. For some reason #4 seems to be more available than the others.
I started with #1,3,4. Then I got #2 (the most expensive) and then finally #5,6.
I have been told I should insure them for about $3000 USD.