In most cases, absolutely nothing (or close to it). Generally people take PA jobs as a foot in the door, and will take whatever pay is on offer.

filmmaking.net

Actually, that's not altogether true. PA's in LA, on a commercial or on a typical "Hollywood" movie with one of the larger studios earn somewhere around $200-250 US. That's usually "flat", which means no matter how long the day (and your day is longer than everyone elses), that's all you get. No overtime. There are various levels of PA-ing, and so pay is also dependent on experience. Most PA's move onto other jobs before pay increases make a difference. A key (or head) PA, who is in charge of all other PA's will make more and have more authority. Generally this person also has more respect amoung the crew and is thought of as the go-to person for all the little things that just wouldn't happen otherwise. These people are usually trying to get enough hours to qualify to enter the DGA as assistant directors where the money ain't so bad, and the benefits actually exist.

By the way, that $200-250 is per day, and though it might seem like a lot, when you're first starting off, you might work one or two days a month, if that... and the days are looooooooong. 12-16 hours are typical, and if you're working 16 hours, you're not getting a lot of sleep.