A Phase I environmental site assessment is a records and observation study, done to the ASTM E1527 standard. The consultant pulls historical aerial photos and old city directories, runs regulatory database searches for spills and underground tanks on and near the site, interviews people who know the property's history, and walks it. Nothing gets sampled. No borings, no groundwater. If the Phase I flags what's called a recognized environmental condition, then you're into a Phase II, which is where sampling happens and where the price jumps to something like $15k and up depending on how many borings and wells they need.
Both quotes you heard are plausible. A clean, small, single-building commercial site commonly runs $2,500 to $4,500. A site with fueling history nearby, an old dry cleaner in the neighborhood, or several structures runs higher because the records work is heavier and the report has to address more.
The reason the lender won't move without it: a properly done Phase I supports the "all appropriate inquiries" element of the federal landowner liability protections under CERCLA. Whether those protections actually cover your situation turns on the facts and on state cleanup law, which varies a lot from state to state, so that's a conversation with environmental counsel rather than something to assume from the report.
Two practical points. Timing matters, because the report generally needs to be dated within a year of closing with parts of it refreshed inside 180 days, so ordering it too early means paying twice. And have it addressed to your entity, or get a written reliance letter. A report the seller commissioned for their own bank doesn't do you any good. Ask your lender for their approved consultant list before you sign an engagement letter, since some won't accept a report from a firm they haven't vetted.
