How much does it cost to deal with i've got a tree growing over my septic field?

The cost driver here is the ground, not the tree. No crew can drive a bucket truck, a crane, or a stump grinder across a drain field without risking crushing the laterals, so the work reverts to climbing and hand-hauling, or gets staged from a distance over ground protection mats — all slower and dearer than the identical tree in an open yard. Stump grinding over a field is frequently refused outright or limited to a shallow grind, because the grinder wheel and the lateral lines occupy the same depth. Have the field located and staked before anyone quotes it. And note that removing the tree does not clear the pipe — that is a separate septic contractor's invoice.