Openreach aim for a cost per property passed in the range £250-£350 according to
this article. That's across all properties, SDU and MDU. Therefore, what you calculate is in the expected ballpark.
In addition, the cost of connecting an individual property also around £300, although there is a £100+ charge back to the service provider for activating a line.
Yes, it will take quite a while to pay this back. WIth Openreach they can expect a very high take-up rate in the longer term, as they'll be mostly migrating their existing copper customers onto it. But for the same reason, that's why there wasn't a business case for upgrading before, since not many people take high speed services. It's only the competition from altnets which has forced them to do so, to avoid losing customers.
Ultimately they'll have a simpler and more reliable network though, and save money through the much lower fault rate.