Surely the cost of that (having several masters) is negligible/non existent when you're talking about digital media?
Not for Blu-ray or DVD.
You have the cost of authoring the distinct version of the disc, the cost of QC'ing it, then the cost of replicating it.
Every region coded master has to pass full QC and if you're region locking them you typically try to put region appropriate trailers and additional content on them.
You effectively end up paying twice for masters (and QC on them) to use, which is usually several thousand pounds* assuming you don't introduce any errors, whilst potentially losing some quite hefty bulk replication savings**, even if you don't do all the replication in one facilty you've saved the cost of the mastering and pre replication QC.
It's much the same reason many DVD's were produced that could be sold anywhere in Europe, often with all the necessary ratings logos on them (although sometimes they did separate screen prints for the discs as well as inserts for the cases).
Some of the DVD companies have been doing this for 20 odd years, as pal 50 meant that it was often a big money saver to make masters that could be used for both the UK/Ireland (r2) and Australia (r4).
*I think simply getting the master physically made (no authoring or content changes) is something like £500-1k.
**There are some very significant savings to be made at certain points in the replication process, for some small runs for example it can be cheaper to do 1.5x as many as you want and potentially scrap a large portion of the run, for really big runs the cost saving is likely to be less per unit, but even if it's just a penny or two over a run of say 5 million discs that's a lot.
