A year ago, or maybe even less, everyone was saying that there wouldn't be any new DMUs because
- it was impossible to fit a Euro 6 engine into the British loading gauge, or at least so difficult and expensive that it wasn't worth trying for the limited number of units that could be sold;
- no ROSCO would finance new DMUs because electrification would reduce the opportunities to use them after the first few years and they might not get their investment back in the longer term;
- they were so much more expensive to buy than EMUs.
But now I read about "cheap" DMUs and the "bulk purchase" of new DMUs. So what's changed? Yes, electrification will be slower, but that won't alter the problem of fitting in Euro 6 engines - if that really was a major problem; or reduce the cost of DMUs; or create a full 30 -40 year life for a "bulk purchase".
It appears likely that some of the stuff being said quite recently was badly informed, or that some of the stuff being said now is badly-informed, or maybe both.
In the meantime, it seems to me there is still a case for an economic product like the D train for use where higher speeds aren't needed and which doesn't have to last for 30 years or more in order to cover the cost of procuring it.
I don't recall there ever being a consensus on these forums about DMU procurement, so claiming 'everyone' said something is rather bold. Some of your other points are also overstating - or oversimplifying - what has been discussed.
-It would be very difficult, if not impossible, to adapt
existing designs to Euro IIIB emissions standard. The implication is that the next order would therefore need to cover the costs of a new design; this presents particular problems for small orders.
-An illustration being when TFL tendered to extend (or replace) their eight 2-car 172s, the prices were unpalatable and no order was placed.
-ROSCOs rarely finance any new trains without some form of underwriting, usually a Section 54 undertaking. This essentially means the DfT commit that the units will remain leased beyond the current franchise, to ensure the ROSCO is not exposed. Otherwise risk is priced in, and things become unaffordable. An example taken from the West Coast franchise consultation:
The core fleet has section 54 undertakings guaranteeing use of the Pendolinos until 2022 and of the Super Voyagers until 2016
So, while a ROSCO may finance a popular unit without backing, such as Porterbrook's speculative 387 purchase, DMUs are only likely when the government a) specifies and b) underwrites the order.
-Price we don't really know, there haven't been any orders for 5 years! The CAF order is £490m for 281 carriages, 140 of these are DMU. The GatEx 387 order was £145.2m for 108 EMU carriages. If we say the CAF EMUs are priced head-to-head with Bombardier at £1.35m/car, that makes the DMUs £2.15m/car. In comparison, the London Midland 172s were £1.35m/car, five years ago.
That cost is close to Vivarail's estimate of £2m/car, which they claim to provide a 3-car train for.
I don't see this as a bulk order; it's probably the minimum viable order for a new design. As to whether it's cheap, or any more orders (bulk or otherwise) are in the pipeline, I guess we'll continue to speculate.