Cheaper (financially) initally, quite likely. BUT in the long term it will be enviromentally more expensive, and almost certainly more expensive financially too. In other words, sticking diesels in IEP and leaving chunks of the IC network out of the electrification program would probably be a lower capital cost, but very much more expensive in operation.
Even the capital side is not as clear cut as it would seem, since every route IEP will run on needs to be cleared for 26 metre coaches. The main sections of the GWML and ECML (being mainly straight, and in the GWML case orriginally built for broad guage) will probably be quite cheap to clear compared to the more sedate lines. My suggestion is that alot of the less 'mainline' routes are not cleared for IEP, with IC125s, IC225s, Great Western's small class 180 fleet, class 22x trains (converted to bi-mode by adding pantograph cars*) and perhaps mark 3s from retired IC125 sets working with DVTs and new locomotives, being cleared already, used for these services instead.
For the ECML, Hull is under consideration for electrification**, as (I beleive) is Harrogate. If both happen, that leaves one train each way per day to Lincon. Just drag it with a class 67, you don't need to buy diesel engines for a fleet of 35 5-car bi-mode IEPs. Same story for Aberdeen/Inverness, where you can share locos with the sleepers so don't need many additional locos allocated to the area.
*The fact you need these extra vehicles, and probably Pendolinos to displace some from ICWC, is the main reason the no IEP bi-mode option is more expensive. If you kept IC125s (with a fair bit of under the wires running) on a fair few routes instead, wiring to Swansea and perhaps Cheltenham could be mostly paid for through the savings gained from not putting diesel engines in IEPs and not having to clear un-electrified routes for their long coaches.
**As part of TPE electrification project