That depends on a lot things, so it is hard to answer. To get the most bang for your buck, regardless of what level skier you are and when you are on a mountain, go for a private. Otherwise, you can try to play the odds and get a private or a semi-private lesson for the cost of a group. For example, if you are an upper level skier - making parallel turns all the time on all terrain - you could sign up for a group lesson at 1 in the afternoon on a wednesday and probably be a group of one. Sign up for the same group lesson on Sat. at 5 and you will be in a group of 6. At the lower levels, chances are there will always be somebody else in your group lesson unless it is the first lesson time of the day.
Private lessons are probably worth the extra money on a weekend, particularly from the middle levels of skier on up.