Wal-Mart is going to be your friend. They have some very suitable plastic plants that are soft enough to use with a betta -- you might need to trim some edges with scissors. One is a largish amazon sword plant ($4) and there are others that are just labelled as small plastic plants. Look for ones that feel nice and soft, and don't have sharp bits.
You can get aquarium salt there (Jungle brand) for about $3 and also a glass thermometer with suction cup for $1.50.
A good adjustable heater will run you close to $30 and you can't get that kind at wal-mart but they do have non-adjustable ones for about $17. Not ideal, but they will do.
As for tanks, you can get an AquaCulture 360 degree view aquarium (3 gallons) including light and undergravel filter for $30. Just make sure to examine it carefully before you buy because many crack in transit. There is also a two-gallon version.
In all truth, the best water conditioners (like Prime) and foods are available at places like Petco and Petsmart, and they are fairly cheap.