Separating this question into two pieces, as it's really two separate things.
- Is it okay to not tip for something that is expensive, when tipping is customary?
In general, no. Tipping is separate from paying for the item, in particular for services where the worker is expected to receive remuneration primarily from tips. Waiters and waitresses, for example, may be paid as little as $2.13 an hour, expecting to make the remainder of their wages in tips. The price of the food goes to the owner of the restaurant, and the tip goes to the server, so deciding to tip less/not at all because the food is overpriced punishes the server and rewards the owner.
In fact, better servers go to more expensive restaurants because they can expect a higher tip there - thus ensuring you get higher quality service at better restaurants. To the extent the market works, you get the service you pay for in each location.
When you're deciding where to eat out, you should factor the cost of the tip into your choice of restaurant - if you go to a place with $20 entrées, think about it as being actually $26 ($20 + $4 tip + $2 tax).
- Is it okay to tip my hairstylist less than 15-20%?
Hair stylists are typically tipped similarly to other personal service employees - 15 to 20 percent. See for example this recent Today article; they recommend a minimum of 20%, though they report that it certainly varies. TripAdvisor recommends 15-25% as well.
If you're a repeat customer of the same salon/hairdresser, tip well: you're probably going to want special service at some point (emergency appointment before the big interview/wedding/party/something; reschedule a last minute cancellation; etc.) and being a good tipper is important.
This partially depends on where you're getting your hair done, of course. Remember that many salons are effectively just providing the space, and hairdressers rent chairs; they're probably not taking all of the money you're paying them, as much of it goes to rent that chair (in NYC, that is likely thousands of dollars per month).
If the cost is of a concern, choose a less expensive hairstylist or go less frequently. It is possible to go less expensive; look in the poorer areas of town, you'll find lots of lower priced hairstylists. And if that doesn't work for you (or you can't find them), go every 8 weeks instead of every 6 weeks if the price is an issue.
So to sum up: tip your hairstylist, and tip then around 20%.