Fat Dragon Games has it's Caverns of Chaos as the new Deal of the Day at RPGNow. E-Z Dungeons is probably the most affordable terrain that is useful and pleasing to the eye. Print and Play makes this an infinite series of caverns.
Caves of Chaos is usually $12.99, but for the next 24 hours or so it can be grabbed for $3.90.
From the blurb:
Caverns of Chaos provides you with a completely modular cavern construction system. Easy to build and featuring lush graphics, this set includes flowing lava falls, a modular bridge system that includes our exclusive no-slip construction system that allows you to put miniatures on inclines, a massive dragon skeleton, stalagmites, tiered terrain, and much more.
This PDF includes:
Modular cavern walls (1", 2" and 4" lengths)
Modular bridge and ledge system, expandable
Modular elevated floor models (6 styles/heights)
'L' wall intersection
T' wall intersection
'X' wall intersection
Cavern wall opening 2"
Cavern wall opening 4"
Flowing lava fall
Lava tiles (multiple styles)
Water tiles (multiple styles)
Crevice tiles (multiple sizes)
Massive dragon skeleton
Exclusive no-slip construction technique that allows miniatures to be placed on slopes
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