It is hard to pinpoint exactly why one price is higher than another. As a third generation underground contractor I can elaborate on a few considerations that any underground excavator must consider when pricing a project. The fact is that anytime you excavate either underground or under a slab there is inherent unknown risk (hence the fact that the work is underground and as such not visible for inspection). A qualified local contractor will usually have some idea as to underground conditions in your area (i.e. high groundwater table, possibility of rocks or boulders at excavation depth, shoring needed to meet OSHA requirements... the list goes on and on. A successful contractor must mitigate for these variables via risk management, which may include risk premium and/or contingency funds. Any contractor that performs excavation services and does not include risk planning and/or risk premium will most likely not be in business for long. Bear in mind that any type of excavation is risky business, both for health and safety reasons as will as unknown time/cost considerations.