Only the heaviest grinder is worth the bother. Too light a machine will still mess up the ground, turn good soil into unusable mulch, leave enough stump for honey fungus to first yellow the lawn for years then over-green it unfixably. Been a gardener 35 years and how depressingly few times have I seen a stump ground right out. I'm pretty sure the pricing question is easier once you've bitten the bullet and signed up for a competent machine and some track to protect lawns. The cost of depreciation or lease of your gear will dictate how you price its work. Cost + profit per year / a realistic no of days / no of stumps quoted for that day. You certainly can't go measuring each tree and squinting at how near it is to some fence or wall and try to apply a five-stage multiplier. you'll go nuts and more importantly you'll forget your method and misquote yourself. Always assume the worst, be prepared to lose out over a quote and move on, and above all keep it simple. Anyone who knowingly goes for the cheapest will frequently have bad work done and will not often spread the word that your more expensive proposal was overpriced, maybe the very reverse. Best of luck to you!