I've stopped thinking so much about estimates and shoot from the hip onsite while trying to account for all the variables, but I'm not relying on my income to support a household, which makes things way easier. On the lighter side (has to be a good friend/ close family) I try to put at least 300 in my pocket after expenses. I don't always hit that, but many days I blow it out of the water. I do work for a contractor friend for 25/hr, but its always one of HIS jobs and I'm not on the hook for damages (also my reputation isn't on the line as much). Otherwise I climb for 75/hr for friends and family, 100 everyone else, I figure 100/hr me and a helper working on the ground, ballpark 600/1200 for me-helper-chipper for a half/full day.
Doing this kind of stuff on the side, without having a business set up and insurance and all that, I wouldn't worry about undercutting a "Legit outfit", you're just helping your buddy with some yard work. I've had friends ask me for a price, and told me that had a lower price, no hard feelings on my end.
If a customer has a "wishlist" - depending on the cost, I'll price the priority trees, then add a "while I'm here and set up" price per tree until they reach their ceiling or realize I can do all the work they wanted without blowing their budget.