first question, what fish do you have in that tank?
As for expense, it really depends on what you get.
To say that sand or gravel are better is a little generic, it really depends on what you have in the tank as far as fish are concerned and as well what if any live plants you have.
If you are looking for a good sand to put in a regular tropical community tank then I've often heard a medium to course grade River Sand recommended. Tahitian Moon Sand is really nice but IIRC it affects you water chemistry. Some people even use the sand used in pool filters. There is nothing wrong with that either.
for the expense, it is really a matter of what you get. You can pay through the nose for it or you can get it quite cheap, just depends what you get.
