I'm not a fan of this practice. The thing is that in a saltwater tank, the substrate is not just there for visual aspects.
It forms a part of the water chemistry - I.E. buffering. If you just leave it out or replace it with something that is not calcium based (like sand), unless you have a forkton of live rock, you may have issues keeping your pH stable, not to mention the difficulty in finding sand that doesn't have God knows what in it that you don't want.
I can tell you from close, personal experience that unfortunately with a tank the size of a 180g, especially if it's a reef, there is very little that comes cheap. By the same token, that makes the investment in proper substate a very small part of the total project cost.
Think long and hard before cheaping out on this one.