Luke, rember 55K is in dollars. It is also twin IR instruction which is the upper end of the instructor salary range. In the UK the same job pulls in:
SFT £25K+good bonus/profit sharing.
Oxford £25K+bad bonus.
Pilotbear, Sadly these figures arn't for you. In the past an AFI (sorry its now FI(A)R)sat around all day, washed the owners car, flew a few trial lessons and then took home £4/5 flying pay in total. £Aprox 2/4K a year. Not much better for QFI's.
Things are getting better. Enlightened clubs/owners pay a retainer - £2/15K plus flying pay £4/15 per hour. The really good outfits will give you this for supervising solo. I wouldn't fly for anyone that doesn't do this, after all they charge the training rate, you have to do the work so they ought to pay you. Also your licence is on the line.
Avoid flying for an outfit in the middle of Wales with only one runway. Unless you like being shouted at for not flying VMC exercises in IMC. Pay is bad as well.
PPL instructors should start a league table so it becomes known who it is worth working for. It would also get the hyenas into order.