If you're just planting the trees then you will be exposing yourself to very little liability. However, if you have to warranty or maintenance the trees in anyway after install, you're pricing structure would probably change quite a bit.
You'll be a lot faster planting them with an auger rather than a digging with a mini ex.
If it's just an auger (in soft soil), plant and go operation I'd figure somewhere between $75.00 - $155.00 per tree.
Design - Build - Restore