It more or less comes down to use, like call volumes. Since every piece of equipment has a maintenance level at certain milestones (time or actual mileage in the case of vehicles), the larger departments will hit those milestones more often. I.E. use/abuse their stuff more often in the line of duty versus sitting around waiting for stuff to happen.
In the average department where there are 400 total calls per year there is no difference in wear/tear and maintenance on the two types of chassis, especially considering that many calls a different truck rolls on it. The overall view is also that the cab only gets used for 5-10 minutes on the way to the run. The rest of the components of the truck are the same (pump, tank, compartments, etc) and that's what used the whole call. So since the main argument behind custom is the cab, there's no real benefit to something being used for an extra 5-10 minutes per call.
Obviously there are nuances to it, such as max HP since customs can take the bigger motors. And if you have a solid response 24/7 where you need 6-8 seats, it makes sense to have custom. But at the same time, look at the cost-benefit from the other side of the coin: for the same $500k they can give a truck to 1 department, or fund 2 for 2 departments which may handle more runs and cover a larger combined population, meaning a better investment.
It also depends on the truck. We hit the 12th heavy rescue ever in the program last year (our 3rd) and that was $350k. Quints and aerials are all custom chassis but that's a moot point since no one really builds them on commercial chassis. Pumpers and tankers, WUI, and the like can be built on either one but dollars to donuts the custom costs a chunk more with no inherent benefits like I stated in the previous paragraph.
So in a nutshell, there really isn't a way to get funded for one. Of course there will be someone that comes out and says they hit one, but 1 out of 1000 apps asking for a custom chassis pumper isn't much support in favor of making that application a competitive one. I haven't done the math to go, but I'm pretty sure that most everyone will agree that over 95% of the pumpers and tankers funded are commerical chassis requests. Most end up funding the difference to get the commercial chassis afterwards by adding local funding, but just look at the spreadsheets from the past years, most fed share amounts are under $300k meaning the project request is $325k and under. The changes in standards, raw material costs, and other factors are why it's even possible to put in an app in the low $300k range. 2-3 years ago there way no way to be competitive over $300k.
Plus it depends on who you ask for pricing from. I've had several folks in the past couple of years get 4dr tankers with 2500+ gallons of water for under $250k excluding equipment. Others swear they can't get one for less than $330k yet every year I see about $100k in difference from the low to the high quotes on the same exact specs. We still have folks getting 2dr tankers for under $230k, some under $200k. There's BMW, there's Chevy, and then there's the realization that a Cadillac is a Chevy with a $20k higher sticker price and a different set of badging glued on it. Not saying they aren't nice, but does anyone do more than $20k worth of difference in their commute in 8-10 years?

