What's made IEP even more expensive is that the costs include maintenance and full availability (i.e. Agility will get fined if they do not provide sufficient "fit" trains for each day), which makes them even more expensive than the basic ROSCO lease.
This could make the overall cost to a TOC cheaper (compared to keeping the maintenance "in house"), but it makes the headline figure look larger.
Continuing on this theme what is rarely understood is that PFI contracts general represent full risk transfer from the operator to the service provider over the total contract term. This may as you point out make the service actually cheaper in overall cost or may not but it does result in a very high apparent cost as all costs have to be quantified and considered over the lifetime.
Equally the prescriptive nature of PFI deals will normal prevent the provider from sweating the asset as can be and is generally done with traditionally procured assets.
To provide an example my employer was recently involved in provision of a number of new schools for a local authority some where to be funded traditionally by the LEA some under a building schools for the future PFI deal.
The PFI schools all had a build value per pupil much greater than the traditional schools needless to say the local press figured out that the PFI schools where "more" expensive. However what wasn't ever publicised was the build specification.
The PFI schools where on a 30 year term and thus the provider focused on cost over the total term, the LEA had cost focus primarily on build cost without really factoring operating costs for the schools they funded directly.
To better illustrate how this played out the roof systems available came with a number of warranty options of 15, 25 and 50 years. The PFI scheme costed all three including a in contract replacement factor for the 15 and 25 year warranty systems end conclusion the 50 year life system was cheaper over the term and was thus installed. The traditional procurement schools also costed all three options but only on the basis of installed cost, maintenance and replacement where not considered at all needless to say the 15 year system was selected.
Similar exercises where done on cladding and glazing, needless to say we expect a contractor be back in 20 years to replace everything on the traditional build.
I'd wager that if the PFI contract rubbish was stripped away from IEP the build cost per vehicle would be no more expensive than a traditionally funded vehicle built to the same specification.
As service available is the responsibly of Agility trains I'd expect Agility will either be procuring more operational spares than would normally be needed at their cost or the trainsets will be overbuilt to increase reliability.