They will be expensive to start with and not so further down the track.
I wouldn't count on it. I've been using DVD-RAM for perhaps the last ten years for archival and blank media still costs perhaps 5x similar DVD+/-RW.
When I built this new workstation I'm using now a couple of months ago I did pay over the odds for an M-DISC capable bluray drive with the single layer discs in mind, but when I looked at the cost of media I decided that I'll stick with DVD-RAM for the time being. It doesn't help that there seems to be a pitiful lack of competition at the distributor and retail level so the most attractive cost/disc prices have ridiculous shipping fees attached to them.
The savings come with volume and it seems there is little mass market interest in long term integrity - you can see some of that even here, with all the attention focusing on the potential problems of reading in centuries time, but completely failing to grasp that 99% of the archival market is more bothered with readability after perhaps 20-30 years, 50 at the outside.