Quite a lot of the cost is the NIMBY appeasement mentioned upthread. Something like a quarter of the line will be in tunnels. Making a slower line wouldn't make that any cheaper.
The extension to Euston was supposed to have 11 platforms. Even the reduced scope now being implemented is 6 platforms, I believe. All 11 were required to handle the eastern leg of HS2 [providing bypass capacity for the East Coast Main Line out of King's Cross and the Midland Main Line out of St Pancras], and services to Scotland and Manchester [bypassing the West Coast Main Line from Euston's classic platforms].