Paul over at Paul's Gameblog put up a post about tools in D&D:
Tools in D&D
He poses a good question for D&D games - what technology level is your game?
With GURPS, this is much easier. Variable tech levels as a concept is baked into the basic form of the game.
Even so, what tool are available wasn't always an easy thing to answer, especially in the early days when digital search wasn't that great and resources just weren't up online.
These days, I just say, GURPS Low-Tech is the place to look.
Before the first GURPS Low-Tech came along, my go-to for equipment in my campaigns was . . . And a 10-Foot Pole by I.C.E., for Rolemaster. I found it a great resource for relative costs, weights, and availability by technology level. If you can find it, it's a useful resource. It's much better if you're playing Rolemaster of the same era, but either way, it's a very useful book.
After the first edition of GURPS Low-Tech came along, I used that. The next edition, the one I helped write, was much easier to use.
These days, that's where I go. For my DF game, for example, things are in the low-TL4 level of technology, augmented by superscience tech and magic. In my previous game, it was a solid TL4 minus gunpowder. I think it would be useful for a D&D GM, too, but I'll admit I'm biased in favor of the utility of GURPS books.