Probably depends on where you are.
I'm in S Beds and mine charges #35 (sorry pound sign not working!!) I've been 3 times in 4 years (all for child-related neck/shoulder pain) and each time I've had 2 sessions.
Live in London, also about £35 plus for a fully qualified osteopath. But you could try a school of osteopathy - cheaper, and for me has usually been a good experience (the students are quite far into their course, and the teachers are often brilliant).
I see an osteopath for neck/shoulder pain, and they charge £38 for 45 minutes (London). They're very good about saying if another session is needed or not - and I always know if I need another one anyway!