Are you seeing any problems with your corys and the gravel? I thought the corys don't like gravel thing had been debunked as a myth. As long as the gravel isn't too abrasive it shouldn't be a problem. I keep corys on standard pets at home aquarium gravel just fine. I do find pets at home sand to be too fine and powdery though.
Caribsea substrate is intended for marine aquariums. Its calcium carbonate, and will slowly disolve and push your pH way up.
Most fish stores will have a good selection of sand substrates, much more cost effective than specialist substrate products like caribsea. Go and have a look at what's on offer. Unipac make a variety of sand substrates, and there is good old play sand that you can get from Tesco.