Stone, brick, cobblestone, wooden plank floors are pretty cheap and easy to do at home. Put some putty on the base, use a toothpick or a hobby knife to form some crevices. You can add texture, if desired, with some sandpaper, slate, bark, jagged broken pieces of wood or anything with some texture, just rolled over or poked into the putty.
If you buy a textured sheet, from Plastruct or Evergreen or where ever, you can just push the putty on the bases into it. Or you may need to make a negative from the sheet, then push the base into that...either way, it's pretty cheap to do a whole bunch of models, and WAY quicker, but the cost is on the high side, for just a couple models, compared to just buying bases.
You could also cut up cardboard (cereal box) or balsa wood or plastic sheets (plastic card, blister packs) or whatever into long strips/boards (for wooden planks), short boards (for bricks/stones) or irregular or round shapes (for cobblestones). Glued onto the base and painted up right, they can look pretty decent too. Easy and as close to free to do as is possible if you use a cereal box, blister pack or similar for the material you cut up.