Hi JeanLady,
Not sure about the price difference, but can highly recommend Dr Heather Eade. I have recommended her to a few friends who have food intolerances and all of them found her to be very thorough. I see her for other issues and also find her to be very well informed and conscientious.
The one I had is much cheaper as it's non-intrusive, it's just a pen she touches to your finger. The one my friend had was something like 2000 'cause it was a blood test. Dr Heather recommends the test she thinks will best diagnose your problem. It's worth it, IMO. I could go on and on here, but have already on other threads :)
Organic Cafe does food intollerance testing. They might be cheaper?
http://www.organicfoodsandcafe.com/index.php?option=com_content&view=article&id=85&Itemid=85
I have looked through some threads and see a number of people have done this through Dr Heather, I just rung up and have been quoted over 2500, others I understand have paid 1100? Can anyone clarify at all? TIA