IMO, the crew is not responsible for profit. The crew's job is to go out and do what can be reasonably expected of them, in style and in good spirit, ideally. The business is responsible for profit. If a salesman is underbidding and the crew can't reasonably "meet the profit margin for the day", that is not the fault of the crew.. If the salesman is overbidding, and profit is over on a job, that really isn't the crew's doing either, and I see no reason to reward them for it. Coming up with an incentive plan to get people to do more than can be reasonably expected, sounds unreasonable.
I run a 4-5 person crew that I am typically present on. I have a contract climber 4 days a week (2 different guys each two days/wk), a really hard working ground guy/novice climber/mechanically minded individual capable of fixing many things...., and one to two less seasoned ground guys. I don't expect anyone to do anything that they aren't comfortable doing, and typically take care of the stuff that no one else wants to do... It works out very well and aside from the occasional issues, team moral is typically high.
As climbing monkey said, people want to like what they are doing. Real hard working folks will feel that the bonus is the fun that was had working as a team and getting things done in style and good spirit, particularly if they know they are being compensated well for their efforts. If they think they should be making more money, or that an unreasonable amount of work has been put in front of them for the day, team moral is not going to be good. As a boss, you should ask yourself if you are fairly compensating your guys. You should ask yourself if your guys get along. Is there a bad apple? Moral is everything, and sometimes you have to get rid of the people that are messing with it. Often the boss is the one messing with it....
Great workers should be paid well and this behavior should be a baseline. The idea of expecting a bonus for hard work just doesn't do it for me. I do give people bonuses when I know that they sacrificed extra time or effort for the good of the group (ie homeowner adds a few things at the end and everyone is already gearing down as their job was thought to be done. Always a kick in the pants to have to drag your ass up another tree for a branch or two). I understand that this sucks, and they respect me for appreciating it if they agree to help finish off the job. I would never make them do anything. If they wouldn't do it, I would.
I think just paying people well and putting them in a positive "reasonable expectation" environment is the key. Weed out the shitbags and don't expect people to make money for you, just expect them to do tree work. Don't ask for a bonus ask for a raise, and when you ask for the raise, you should have a good reason if they ask you "why?"
A little bit of a rant... apologies and cheers