Intersango now charge 0.6% per transaction (I think) and in GBP they've moved 255,621GBP in the last month - this is approx 1500 GBP profit. Double this for both sides of the transaction and you have 3000GBP in 1 month. This is the second most popular exchange currently....
How can any exchange survive on this or lower
Sorry, you made a mistake there, that number isn't profit, thats called revenue. And because an exchange doesn't run on air those costs have to come out of that revenue.
I believe that the term "shoestring" applies, and that to a large extent all businesses would probably make more cleaning streets or toilets for min. wage than running their own business, at least in the beginning.
You have to remember how quickly the bitcoin economy is growing, about 1 year ago it was valued at about 10K USD, now it's over 40Million USD, that is some amazing growth.