The entry fees and prize funds vary by tournament and venue. The tournaments I have been playing are in chess clubs, which is a $25 for 4 games. But that is just in my section of the world - tournament entry fees and prizes where you are could be different. Consult the chess club that you wish to join to find out how much money you need to save up.
If you’re just getting started, you also have to pay USCF membership, which is another fee of about $50 or so. Hopefully that shines a bit of a light on tournament realities.