A 10-spindle of 6X BD-R discs was recently $5 at Newegg. That's equivalent to a 250GB HDD, for $5. I disagree that HDDs are cheaper than BD-R discs. They may be more convenient, and faster for random-access and re-writing, but they are not cheaper.
A 50-spindle of 6X BD-R discs is $20. That's a 1250GB HDD for $20. The cheapest 1TB HDD that I've seen is $40. BD-R is less than half the price of a HDD.
I bought a 5TB USB3.0 desktop external HDD for $124.99 at Newegg on sale. $125 would get you six 50-spindles of BD-R and one 10-spindle. That's 310 BD-R discs, times 25GB ea., that's 7750GB worth of storage for the same price.