Since I'm in Australia, my charge is not really relevant (it's AUD$50 per hour).
I'll turn the question around though and ask "What's the cost of not engaging an instructor?"
Being mostly self taught in the early stages, for me the cost of trying to do it alone was:
- A 6 month bout of tendenitis
- 3 years of frustration, an elongated learning curve.
- Countless metres of knotted tippets and leaders
- Many "blown out" sessions, where any wind from the "wrong direction" forced me off the water
- Who knows how many rods, lines, reels and other gear purchased, thinking I would cast better if I just had the best gear ...
If I could have had lessons, I would have. At the time, there were no instructors in the western half of Australia.
My point is that by improving your own casting (with or without an instructor) you will save yourself MUCH more than the financial cost of engaging an instructor. With an instructor, you'll improve much faster than otherwise.
Cheers,
Graeme