Don't be discouraged by the prices of lessons, but don't be naive either. You can certainly start in group classes, but if you want to compete, you will need private lessons. You may want to check out the prices and be sure it will be doable when you get to the point of wanting private lessons.

There are two ways to compete: amateur and pro/am. Amateur is a couple who are both students/non-pros. Pro/am is competing with your instructor as a partner. Competing as an amateur is much less expensive, but of course it depends on finding an appropriate partner. (If you stay on this board, you might run across a lot of debate about which is better to do, amateur or pro/am. Ignore it. They both have advantages and disadvantages and each person needs to decide what's right for them.)

You may have another option. Check with any colleges near you. Ballroom is becoming increasingly popular at colleges and lots of them have teams that provide training and a pool of possible partners. Some college ballroom teams/clubs accept people who aren't students at the college. Also, when it comes time for you to think about gong to college, you might want to check out which of your possible colleges have teams or clubs.

Good luck! If you decide to go for ballroom, I expect you'll love it!