What would be a decent price to pay for a new windshield installed in my '85 D-150 regular cab?

Not sure what they are going for on that era of truck but you may want to do what I did which is call your insurance agent and ask him/her what kind of price you can get for one installed.

I called mine and asked that same question recently and he hooked me up with the people they use for glass replacement and for my 98, they quoted me $259.00 installed.....not to bad IMO.....

Last one I bought was a Pilkington (formerly PPG) and paid $125 installed. Pays to shop around, this guy was the cheapest by far.

Well one way to look at other is if you try yourself and break your windshield while installing your out a windshield. If the break their windshield that they are installing in your truck you still get a Good windshield installed in your truck. Shop around. Even if you have to drive $50-100 miles as long as its cost affective. Tried helping a buddy and said it was a bad idea the whole time. Guess who ended up having to by another Rubber seal and a new windshield. Not me lol. Other cracked and the shop told him we screwed up the rubber. I'll never try it again.

Last one I got replaced on my 74 D100 was $185 with a new rubber seal. Like was said shop around for a good price. I found a place that sold auto glass to other shops. They were by far the cheapest and did it in about and hour at their shop.