"with 100 bets you would win 5% of your bank roll.
This is a good return?"

For most pro's this is a very good return. Definately enough to make ALOT of money from. Remember most pro's make quite alot of bets. I know guys who make 100+ a week. During the busier parts of the year I might make 60-80 bets per week.

The idea behind an advantage of 5% is to just have massive turnover. A lower advantage but at a much higher turnover will make more than a high advantage with low turnover.

"Lets say you have a $500 bank roll betting 1% units."

Pro's will base their bet size on:
1. How aggressive they are.
2. How large their advantage is.
3. How many bets they place.
4. How much variance they can deal with.

Most pro's bet 0.5% to 2%, but they usually underbet by quite a bit (ie bet quite a bit less than the optimum amount). Most pro's are conservative, deal with advantages from 2% to 5%, place alot of bets and can't live with large variance. This is why they have what seems a small bet size.

"Where do you find the $1.91 price aswel, point starts?"

Yep. In the US, most betting is 'against the spread'. This is point/line/handicap betting in Aus. Bets are usually at $1.91
(-110). If the average bet is at 1.91, you need to win 52.4% to breakeven.

A more reasonable example might be a pro with a bankroll of 100k. He risks 1% per play (1k). He makes 1500 bets per year, hitting 55% and so goes 825-675 for +75 units. Ie he profits 75k. If you factor in things like middles, bonuses and low vig (average bet of better than 1.91) then he
will make much more.

It can be much MUCH more profitable than in the example, your only limit is to how well informed you are.