Actually I was talking about just the cost of the tape hardware. Blu-Ray burners seem to be a few hundred dollars
A while back I wanted to search for some old stuff. Not a single CD 9 years or older was readable anymore. And they were all brand name CDs, not the bargain bin spindle kind. Some of the DVD backups I have are also unreadable in most drives after 5-7 years. All the 2x or 4x kind, conservatively written.
I don't believe that blu-ray will fare better. And considering the lack of adoption, it's a poor bet for long-term archival. Think Beta vs. VHS.
Tapes suffer from similar problem - they are not widely adopted. If running an enterprise with long-term support contract, then perhaps. But very few places actually plan for 10+ year archival unless required by law. 2+ years is a luxury most of the time.
Since then I've migrated everything to disks, keeping in mind that as they will fail the data will be need to be periodically copied to new ones. Still, with network, it's considerably more convenient than swapping disks. Eventually I'll probably migrate to the cloud, Amazon is likely to be around for a while and prices are quite affordable, especially for the important data that actually has value.
A 1TB external drive is ~$100. Buying one of these every 6 months, migrating data between them is quite secure. Keeping them connected also detects any failure soon, so as long as there are several copies, it's really not that big a deal.
Considering real life span of data and its usefulness, disks make for a very optimal choice.
Another factor - interface. Tape drives often require proprietary drivers. Oh the jots of trying to get the Win 98 drivers working with NT for a vendor that was bought out and discontinued that particular model. With USB or even IDE you're pretty much set for quite a while still. Plenty of people are using 7+ year old computers for work, so both of these should be easily available in 10 year time. Even if not, old discarded hardware using any generic linux distro will be able to read them.