there are some, but they are really hard to find, and even more expensive, over $300.00 is i remember right
You know for a tank that is 100+ gallons that's really not a bad price if it looks right and you're happy with it.
This summer I plan to start my own business building custom fish tank scenery that can be accompanied by submerged LED lighting and bubble volcanoes. I'm not sure what the exact market is for this stuff, but I should have some examples and some professional pictures of the examples here in a couple of months. I have to move first...that is what's holding me back. I hate moving.
Working with some of these materials can be complicated, but the goal is to make it look natural and random like it would have occurred in nature. I can sympathize with putting 100's of lbs of rock and gravel in a tank. I have a couple 100g tanks that have a few hundred lbs of lace rock in each one and water changes are a pain. One of my tanks is acrylic which makes it hard to find a place to prop a rock up without damaging the acrylic.
The alternative that I plan to bring to the market will be light weight, look natural (or unnatural if that's what the customer asks for), and also easy to remove for cleaning of the tank. Because it will be custom to the application it is intended to leave the customer with something unique.
All in all the projects that people have done in the past look terrific, but they do take time. The projects also require space, time and a mess that some people do not want to deal with.