Hold will vary from book to book for a lot of reasons. What sort of customers you have, how you treat wiseguys, how soon you open, how and when you move your lines, what kind of action you take, your business model etc..... Hold will be much higher in a shop like SIA then it will be at CRIS for instance. While it's important it's certainly not the only thing to consider either. I'd rather hold 1% on a billion than 10% of a million.
Average in the old days was about 5-7% on straight wagers and between 10-20% on parlays and teasers. This was a very small street outfit with low limits. The players didn't have a clue but neither did the guys running the book, LOL. It was a wash almost. Seems high but that's what it was.