My DDs do highland dancing.
Each school will be different but I'll give you our experience.
In terms of cost, I pay $8 per lesson. Lessons are either 1 or 1.5 hours, depending on the level. New beginners have one lesson a week and that increases to three lessons a week as they progress to the higher levels. For lessons, my girls wear a generic black leotard and a studio skirt that one of the grandmothers make for everyone. The major cost is the shoes, called pumps, that are between $50 - $100 depending on the brand. Treated well, they last about a year depending on foot growth.
Once they are ready for displays and competitions, they need a kilt. In our studio, these are usually available second hand or some parents are happy to lend their old kilts. A whole outfit secondhand is about $250-$350. One outfit can last for a number of years depending on the growth of the child. My eldest DD (11) is using her second kilt and I can see it fitting with hem adjustments for another three years still.
As they progress, there are a couple more costumes for other dances. Aboyne, jig and hornpipe. Again, we have both purchased and borrowed these from others in our studio.
As for performances, we occasionally participate in an end of year combined concert with all the other schools in the area but don't put on one ourselves. We usually have the opportunity to perform in about 4 displays each year. These can be for public in parks, or private in nursing homes etc. I believe the senior dancers may have further performance opportunities. None of these displays are compulsory.
My DDs both compete. We have about eight competitions a year. My girls love competing but we don't necessarily attend every comp. We have just started attending some competitions outside our own area as part of family holidays. They are a good excuse to get away.
There are exams just like ballet. My eldest DD is doing her grade 4 exam this year and my youngest is likely to do her pre-grade. Our teachers encourages everyone to do one exam per year right up to their teachers exam.
We are unusual in that we are a first generation dancing family. Most of the children dancing with my girls have mothers and in some cases grandmothers who also danced. That is a non issue and they don't treat me any differently. I have found it to be a very friendly, welcoming and supportive environment.
Hope this helps a bit. My girls both love it.
Feel free to ask any other questions.