I use alot of different substrates and it totally depends on what you want to accomplish with it. By far the cheapest substrate is to go to home depot and get some play sand or tube sand. The trick with that will be to rinse the sand very very well. Start with about 3 inches in a bucket, rinse and pour it about 15 times and you'll have about 1.5 inches in the bucket. That sand then will be safe for your filter and will not cloud your water. If you want to be super serious about your betta though, most bettas appreciate softer water (as do plants) and certain substrates like Red Sea Flora Base, Amozonia or Fluval Shrimp Stratum soften water and buffer down PH. It depends on how serious and awesome you want your tank, a betta will survive fine with sand and harder water, but may be happier and you might experience better plant growth with a flora base.
Oh and to correct my first statement, the cheapest is no substrate, which for me also helps me keep my breeding tanks super clean. Sometimes I do bare bottom tanks and use terra cota pots with flora base substrate and plants. Then a good floating plant like water sprite that dangles a good amount of roots.