For "older houses"??
Perhaps that's a slip, but house windows and automotive windows are completely different, in design, and glazing. All automotive must be safety glazed with acceptable automotive safety glazing, not architectural safety glazing.
For the original poster, $500 installed isn't a bad price. Yes, the door on your 118" is smaller than a door on the other vans. I personally didn't know Ram or Fiat made a sliding window unit for the Promaster yet, only solid ones. The cut out and installation "should" be fairly straight forward, but that does not mean it should be cheap.
We're still considering the R side slider, and rear doors in our '14 136"WB, that came without, and I have to cut through a vertical "brace" across the sheet metal in the R sliding door if we install glass. I am curious if your 118" has that "brace" in it as well, being it's slightly narrower than the other vans.
An FYI for those interested: CRL does not make a glass for the LEFT side slider on our 136" either, though I am convinced it's the same glass size and pattern that goes on the RH side slider. The measurments, angles, and everything "APPEAR" to be identical, though I obviously can't confirm it, at least in regards to trying one, OR that the OEM black ceramic frit on the glass is the same, or is a mirror image of the RH slider glass. When speaking directly to the tech folks at CRL, it was like banging my head against the wall, all they would keep saying was "We Don't Offer a LEFT side slider" when I kept asking "HAVE YOU TRIED ONE yet?"
LOL