You could go for Ballast rather than Slab, which means lower install costs and slower trains, but also means higher maintenence costs, but the headline price will be lower, even though the TCO will be higher and the service will be worse.
Longer trains don't work because you'd have to demolish the towns that the stations are currently in -- extending 200m north/south at every station between Euston and Rugby is not cheap and will piss off a lot more people than building a quiet railway through an empty area, not to mention all the signal problems and points fouling, and what will you do at Euston?
> I'd like to know what incremental cost there is for an increment of higher speed
The big costs are land acquisition and earthworks, which won't change at all regardless of the speed. Euston is another cost, but you'd have to completely rebuild the station, including putting all the platforms underground, for your longer train plan - and Underground space in London isn't that easy.