In my personal experience and from watching my kids I have learned the following:
1. Private lessons over group if you can afford them. Caveat, if you have time to check out the local mountain, see when group lessons have few or no people and schedule a group then. You can often get a private or semi private for the cost of group lessons. Check for deals, some resorts offer ski rental/lesson packages that may be worth your while.
2. Privates are a bit pricey, but the solo attention and pointers to advance your ability really help you experience and enjoy more of what the mountain has to offer. You are going to pay for the daily or seasonal pass whether you only hit the easiest bambi trail or tackle everything it has to offer. Lessons are a tool that helps you access more of the terrain. Its like learning to drive. Day 1 you aren't hitting 110mph in 5th gear, but you don't want to be stuck driving 15mph for the rest of your life either.
2. Be cool with the instructor. If you take the time to be friendly, attentive, polite, and try to get to know them a bit they will really go out of their way to try to help you meet your goals. They will anyway, but I found if you can get a friendly vibe going then the whole lesson just seems to go better. I try to keep the attitude that I am going to spend an hour or two with this person, it's more fun to ski with a friend than an instructional robot.
3. Instructors are people too. At the end of the lesson throw them a few bucks to buy a beer or sandwich later. I find this helps them remember you if/when you have the chance to take another lesson. I always try to throw in a nice tip if the lesson has been for one of my kids. These guys and gals aren't babysitters. They are professionals helping to share their knowledge with my children. Their love of the sport encourages my kids love of the sport. It's a retirement plan, I hope the kids will drag me up the mountain when I am a broken down old man.
4. You are never too old or too cool to take a lesson (I'm not dropping off cliffs, so this may not apply to the truly epic skiers out there). I still try to get in one or two lessons a season and find technique tips and new ideas help me ski better.
5. It sounds like you really love skiing already. It only gets better. There are cheaper hobbies, but how many put you on a mountain in the middle of winter, fantastic scenery, terrain, and opportunity for great times with family and friends? The cost of gear and lessons isn't a factor that can overcome that, only slow the progression.
6. Skiing is fun. If you hit a time when you are too frazzled, upset, or down right out of step with the world, take the day off. Your attitude goes with you everywhere.