There is sometimes a feeling that we yoga teachers should work for love: for the sheer joy of sharing the knowledge and spreading the word. Perhaps that's true, but it doesn't wash with the staff in Morrisons, who always ask for money in return for my bag of food. The fact of the matter is that we need paying for our labours (most of us, anyway). That said, I've never come across a yoga teacher who won't step in at short notice to help out a friend or run a session at a charity do for nothing.
The thorny question of what to charge for covering someone else's classes came up in conversation with a fellow teacher this morning. My advice to her was that it's perfectly reasonable to ask for the appropriate fee for the job; that way the situation is clear and there are no nasty surprises for anyone. This might mean losing out on work occasionally, but so be it.
Go back to the yamas, specifically asteya, which is usually translated as non-stealing. If you undersell yourself, you are giving the other party the opportunity to steal from you, albeit unknowingly and possibly unwillingly.
Asteya pratishthayam sarva ratna upasthanam
To one established in non-stealing, all wealth comes.