Joining the USCF gives you the right to enter any USCF-sponsored tournament, but most of those will have entry fees as well that you'll have to pay. The USCF membership fee pays for the USCF infrastructure, things like keeping track of ratings and titles. The tournament entry fees pay for the cost of the tournament (renting a place to play, paying for tournament directors, etc.) and most of the time prize money.
The cost can vary quite a bit. Near me, there's a club that hosts a couple of tournaments per year with a $1 entry fee and no prizes. The huge, national-level tournaments can cost several hundred dollars to enter and have five-figure prizes available. There's a lot in between too. If you are lucky enough to live in an area with a good chess scene, you'll have plenty of options at any price level.
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