Not sure when I bought my DOpus license but it was a long while ago, for version 6. When you buy a license, you buy it for that version and all it's updates. I don't think it ever expires. You then have to pay another but much smaller upgrade fee to upgrade to a higher version number.

Which I'll be doing shortly, it seems.

And yes, i think it's well worth the cost. This is one of those things I almost refuse to pirate, a great program from a small independent developer amd not some huge software house. These guys deserve your money.