Greetings and welcome to Pokebeach; what brings you to my wall?
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To answer your question about how prereleases work:
Prereleases cost $25 for an entry fee (most premier events are free), but that is just because all the materials required for playing will be given to you.
You'll be given 6 packs of the new set, and from the cards within, you are going to construct a 40-card deck - then you'll play Swiss rounds with matches played for 4 prizes instead of 6. (Rounds are also 20 minutes long instead of 30.)
You aren't restricted to the 4-per-deck rule and all the basic energy you'll need will be provided, but you aren't allowed to trade mid-tournament to adjust your deck. If you play all the way to the end of the day you'll get 2 more packs. In the middle of the day you'll also get a prerelease promo card plus a plastic deckbox.
All in all you're getting about $40 worth of goodies for only $25, plus you have a fun day of hanging out.