To get to a "reasonable" price point you'd need to avoid conflating what would need to happen anyway versus what's new. The main problem I have it that people are throwing the M4, M25 works BAU and changes due R3 together and mixing in T2/2 with R3 and T6. TFL have thrown a whole load of BAU costs under new work as well!
Let me have a proper look at some numbers and I'll try and do a laymans guide and come back.