Power steering problems with my '01 E53

[X5 in CO USA]

All of a sudden the power steering died on my 2001 X5 E53 4.4i while my wife was driving it. She was smart enough to check the fluid level and there was none in it but when she filled it she put in actual power steering fluid and not ATF. When she got home and told me about it the resevoir was empty again so this time we put in ATF like you're supposed to. Obviously I must have a leak somewhere and the car is going into the dealer tomorrow to have the problem looked at; but I know the dealers have problems diagnosing these older model X5's and tend to lean toward very costly complete replacement of systems. I do most of my own maint. and have another E53 parts car so i'm looking for suggestions after the dealer tells me it will cost me thousands to repair.

Any help would be appreciated.

Higgins