The fees for criminal defense attorneys can vary greatly. You can find an attorney to handle your case for $500 and one who will do it for $5000. It really comes down to finding the right lawyer for you and what you can afford to spend. That being said, you can absolutely pay too little and too much for a lawyer depending on your case.
Public intoxication is certainly not the crime of the century but because you are under 21, a conviction will result in the loss of your driver's license for a period of 1 year. As such, you should do everything possible to avoid a conviction. A criminal defense lawyer will only maximize your chances of a successful result.
A lot of times, the right criminal defense lawyer can negotiate the dismissal of the charges in exchange for attending a number of AA meetings. In Orange County, they actually have a one day class you can attend in exchange for the dismissal of your case.
The bottom line, there are a number of options available to avoid a conviction. Your first step should be talking to some lawyers to get some information. If you find one you like, then see if you can afford to hire them to help you.
I hope this answer was helpful.
Unless the lawyer guarantees that she/he can have your case dropped in exchange for some kind of program like Gabriel suggested, (and some can and will) you would be a fool to pay a dime over $1500 for a case such as this, assuming this fee includes all representation up to but not including a jury trial.
With all due respect to the second answer, no attorney can guarantee the outcome of any criminal case. Every case is unique and the outcome can depend on any number of factors.
Can something be worked out that's favorable to you? Absolutely.
I agree with Joe. How much you pay an attorney and what outcome you receive usually have nothing to do with each other. While drunk in public is a relatively low level misdemeanor, you may prefer a more expense lawyer over the less expensive if they help you understand the process, personally handle the case themselves, or make you feel more comfortable. Find the lawyer you feel most comfortable with. If they promise you a result, you should be suspicious.