Cool stuff - terrain is often a really underdone aspect of miniatures. However, pre-made stuff is generally godawful expensive.
I use tiles of hardboard (home depot or lowes, $10 for a 4'x8' sheet, they'll cut it into 1' squares if you don't have a saw), foam insulation (same places, about $15-20 for the same size, 2-3" thick, cut and carve it with a sharp kitchen knife), popsicle sticks (michaels or other craft store, $10 should get a box of 1,000), plaster of paris (again home depot or lowes, another $5 item for a decent sized bucket), paints (any craft store, usually $1 per bottle), foam board (craft store, a couple bucks per sheet, get the matte black stuff, not gloss)
$50-60 should get you the basics for an entire table top worth of scenery. A few random bits of junk around the house yields lots of good stuff too. And a good craft store/train store/etc should have some stuff too, though they can get expensive so I usually just use them for details.
A mostly finish factory:
Large tanks are baby bottles (package of two from the dollar store), pipes are drinking straws, small tanks are empty paint bottles, wood is scraps of balsa with a wood stain, fence is a mesh from home depot (only a few bucks for a roll of several feet), gears are from a broken movement I picked up a local clock repair shop for a couple dollars, girders are styrene modelling bits from a model train shop, cobbles are plaster cast from Hirst Arts molds.
I calculated at one point that my tiles generally only cost around $8 each.
