I think the most cost effective way to start is take a combination of both group and private classes. At a beginner or intemediate level, you can learn the steps and new patterns in group classes for a fraction of the cost of private lessons. You can then use private lessons to learn the proper techniques for these steps and patterns (good dance frame, body and foot positions etc.), and minimize bad habits.

Group classes also give you an opportunity to learn from others mistakes which is very helpful developing good lead/following skills. People who only dance with the same partner usually develop bad habits with one partner compensating for the other's weaknesses. Group classes are also a place to meet new people and possibly some new friends. If you take group classes, I would recommend taking them from a studio just to avoid large class sizes that are typical at University clubs.

Regardless of the type of classes you take, take the time to practice what you have learned. If you don't practice, you will not retain much from your classes. This makes your training a waste of both time and money. Studio's typically offer weekly practice dances to address this.

As you become more advanced and/or if you plan to compete, private lessons become essential. I now only take group classes if I want to learn some new patterns for social dancing.

Blair