I have a DMT XX being shipped to my house from Toolsforu.com. 61.99 and free shipping as of this past Monday. 61.99 will probably be close to the best price you will find for the DMT XX. Incidently, there are two grit combo stones in the dia sharp. They just don't have the holes like the duo sharp, and they are only 6x2 1/4, making them a bit more affordable than the duo sharps.
If you go to the DMT website, find the D6 combo grit stones, google the stock number, say 'DMT D6CX' (coarse/x coarse), you will have several listings come up. The best I found I believe was knivesplus, or a listing on e-bay that I think was myknifesource. If you type myknifesource.com, he will have a link to his e-bay store. Their prices were around 37.00 plus shipping and handling. However they may not have all the grit combinations you need. I wanted a coarse/x coarse and a fine/x fine to have a complete set and it was hard to find a store with both, althoug if you click DMT on knivesplus.com and go to the last page or so, I think you will find them.
I phoned a request to 888knivesrus and he said he could get them for me for a comparable price plus shipping, but when I talked to him later, I was a little unsure if he understood which stock numbers I was talking about. He has sinced e-mailed me and said that it would be late next week before they would come in, and he didn't include a price in the e-mail.