So a 33% or 66% increase in purchase cost ...
Which would have been funded by whom?
This is the bit that we seem to keep stumbling on.
Three coach 185s were okay from day one - especially on TPE North, where four trains per hour replaced three (two coach 158s) per hour (no such frequency increase on TPE South).
The "additional" units became replaced by nine 170s instead - which is the kind of fudging of the numbers that BR used to do a lot - nothing unique to TPE here.
What was the alternative, for the loss making franchise, though? Build them as four or five from day one? Would that have meant frequency reductions? Which place would have lost its hourly Manchester service in that case - Hull/ Scarborough/ Middlesbrough/ Newcastle? Or did your magic money tree pay for 60x 5 coach trains, that are both "InterCity" and suitable for the large number of shorter journeys on TPE?
Bear in mind that this was at the time that Northern and ATW were let on the "no growth" franchises, to put the "only fifty one new trains plus nine cascaded ones" argument into a little context. I don't think that the deals signed in 2002/2003 were that bad at the time, in terms of carriage numbers.
Your problem is what happened once passenger numbers grew. Since 185s were DMUs, the emission standards etc changed (which limited the window for ordering more). And since 185s were bespoke DMUs, the chances of getting a production line opened for a relatively short order became harder to justify (just fifty one extra carriages... plus maybe another fifty one in another few years?).
I'm not a misty eyed nostalgist for the good old days of loco haulage, but at least if TPE had ordered unpowered carriages then there would have been more of a chance of ordering compatible unpowered carriages at a future date (even if made by a different manufacturer).
Ideally, instead of faffing about with twenty seven 100mph 175s and fifty one 100mph 185s, First would have ordered the same 100mph Turbostars that were good enough for lots of other TOCs - meaning that we'd have had a TPE fleet that could have been added to at later dates (either through keeping a production line open later, sharing costs with other TOCs looking to top up or acquiring 170s from elsewhere - e.g. the Hull Trains ones became free once their 222s arrived).
That's the TPE problem - that they couldn't add DMUs easily - not the initial order of 51x3 (plus 9x2) carriages.
There are good things about the 185s (efficient) and bad things about the 185s (ride quality not great, as a passenger, awkwardly located First Class section), but the bigger picture for me is the problems caused by ordering a non-standard fleet of "specialist" DMUs - not the initial numbers and not the grumbles about seat quality.