Even looking at Gotthard tunnel is rather bad because it's just a tunnel though in very difficult conditions and large as you say is a bad comparison against a new railway network under a densely populated city.
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.