[1] https://en.wikipedia.org/wiki/List_of_transport_megaprojects
Crossrail is large in the context it has new stations, depots, extensions, trains, and the very large amount of traffic it is expected to take under a very short construction period.
Looking at the entire picture doesn't make the number seem ridiculously high. If it seems to high you need to look at where it's all going its not just for a tunnel.
FWIW, Crossrail is on this list of the 25 "most impressive" projects - http://www.popularmechanics.com/technology/g2121/the-worlds-... .
https://en.wikipedia.org/wiki/List_of_transport_megaprojects describes it as a "megaproject", and I can agree that everything which is a megaproject is one of the largest construction projects in the world.
Looking at the list of projects which are "under construction", and ordered by cost, Crossrail is #6 at US$25B and the GBT is #10 at US$10B.
Crossrail: 20 million man hours three years ago http://m.londonlovesbusiness.com/business-news/london-transp...
I'd love to see some more reliable figures, though.