I have always grabbed a juice there (over 20 times in the last year) with my Egyptian friends and it has never cost me more than 3LE.
When I went to the khan with my uncles last week, we ate at Egyptian pancakes & got the juice from the guy next door. He came back to me with a bill of 60LE for 5 lemon juice. 12 LE each ! (4 times the actual price !!!!!) He refused to see reason, even when I told him that he was charging us 4 times the price ! (To put it in perspective, a canned aerated drink in the market costs a maximum of 2.5LE, at Fishawy it costs about 5LE and at the 5star Oberoi managed Naguib Mahfouz Cafe in the same area its 10LE+tax. Most 5stars - Hilton/Sheraton/Mariott charge within 15LE for a juice inclusive of taxes)
Fix your price with the juice guy before ordering or don't order at all. I am definitely NEVER EVER going to drink a juice there again !
PS : The pancakes guy has a menu, so there's no trouble there.