I'm obviously not going to reveal actual numbers, but it's safe to they spent less than what they would have spent being on Wordpress for the remainder of the year.
Time to completion is 3 months. No new "feature" development was blocked. Remember - this is a content website (Wordpress is mostly used for content AFAIK and less for building features).
The moment it was done, we just switched nameservers. It had no impact on client's workflow or day to day life.