the price will definitely come down - if you think about the pricing then:
- the Di2 mechs are clearly more complex than the mechanical ones, with the motors built in, so it's reasonable that they should be more £
- the STI may well be simpler than the mechanical versions, as they have switches rather than ratchets, so could easily be no more expensive.
- you have a battery, which obviously costs a bit but not silly money
- and a few cables, which cost very little
so, it's reasonable to think the Di2 prices won't be much more than the equivalent mechanical prices, after a few years. Hopefully.