If you want cheap base rock I can sell you some Texas 'holey' rock that has been cleaned and used in saltwater tanks. It is not as porous as LR/dead coral rock but it is fine to have in your tank.
For the DIY:
Texas limestone is fine to use in your tank. If you find a piece that you want to use check for a couple of things. If it is very red/orange it contains a higher amount of iron. If it is chalky (easily rubs off white on your fingers) it will break down quicker in your tank. I would not reccomend using limestone with either of these characteristics.
To clean and prep the rock for use:
Wash completely with the garden hose or power washer.
Scrub with a hard bristle rock.
Soak in a bleach** solution (5-10%) for a couple of days. Use a powerhead to keep the water moving around the rocks.
Rinse and wash agian with the hose. Let dry completly.
Soak the rock in a saltwater solution just like curing LR. Run a powerhead, make water changes if you want, and the longer the better.
Add it to your tank.
**Bleach breaks down quickly when exposed to air. That is why they use it to treat drinking water.