I'm probably biased, but I think there is a lot of value to getting some formal casting instruction from a good instructor. A lesson will significantly shorten the learning curve.
I don't think the cost of the lesson is as important as who the instructor is. I've seen some poor instructors that charged a lot of money for their "lessons" and I've received some free help from some incredibly talented instructors.
Hey, for the price, how can you pass up a free lesson from Cabela's, Bass Pro, Orvis, etc.? Usually, these are group lessons and the amount of material covered will depend on how large the group is, smaller group size is usually better for the students. But, these classes are a great place to start.
Local fly fishing clubs often have opportunities to learn to cast. They can also help you to sort through your area's casting instructors to identify the better one's if you do want to go the private lesson route (I usually take at least one private lesson each year).
If you've got a good instructor, you will learn more during a private lesson than you will during a public group lesson. But the key is to find a good instructor.