Thingiverse has some interesting stuff, like the Iron Creed trench system.

Terrain is fairly cheap to make without 3d printing. Hills can be carved from polystyrene (24 sqft for ~$10 at a Home Depot). Cut some blocks, stack them roughly to the right height, and then just cut off some of the sharp edges. Coat in PVA glue so you can give it some primer, paint and cover in the model flock of your choice as you would for your dudes' bases. Works for craters too. Back in the day when you could get free AOL CDs, they worked great as a base for craters. Buildings can be made from thin cardboard, cereal boxes, etc.

Aquarium scenery is fairly cheap from the pet store and roughly the right scale. Some neat tropical plastic plants in the aquarium section too. If you've got a general hobby/crafts store, like the kind that sells model trains etc, they generally have model terrain that's reasonably priced, good for getting a bunch of trees on a card base for forests.

Does it have to be 40k, and does it have to be 3d printed?

Back in the day my friends and I didn't have a lot of money to sink into that stuff, so we used to have a couple proper 40k units and fortify them with army men, matchbox cars, and possibly even smaller action figures. Terrain was stacked foam board, lego, wooden blocks, and I think some model railroad structure.

Made for some pretty silly games sometime. Think my favorite one we did was a Maximum Overdrive style game with matchbox cars vs. army men. He-Man may have also had a cameo role as some sort of special unit.

If your primary goal is to just have fun at a minimum of expense, that style of gaming is probably going to be hard to beat cost-wise. Especially if you can find it used.