I had a makeover done at a MAC counter before. I've been told the rule is..if you book it during the weekdays, there is no minimum purchase requirement. For friday-sunday, there will be a $50 minimum purchase.
When you get to the counter, have in mind the type of look you want them to do. I typically choose MAs that have makeup styles that are suitable to me. Some have really bright colors on, some are more on the "neutral/natural" look. If you don't like something, let them know right away and most MAs are really nice and accommodating.
Regardless of whether or not you purchase anything at the end, it's always nice to thank the MA and get their names. Especially if you like their work, some are freelance artists that can be hired for other special occasions.