If you decide to go with sand a 50 lb bag of playsand is just about perfect for a 75 gallon tank after you do a very thorough rinsing and will run you about 2-4 dollars at home depot. Some people prefer pool filter sand but so far I have not had any problems with approprietly rinsed playsand. You can add root tabs if you want to get some nutrients into the substrate.
With that said my next tank will be a full on co2 dry fert high lighting nutrient rich substrate affair. I am definitely happy with play sand so far though.