I think the worst case scenario would be if their septic company says that the field is fine. Then they don't have to do anything and you have to decide if you want to take the chance and proceed with the sale, try to renegotiate, or bail.

The outcome needs to be negotiated, whether they pay, you pay, or you split it. Unless they have a mortgage that will consume sale proceeds after costs, if they're paying for the fix, won't the sales proceeds pay for the repair? If so, the expense would charged to them in the closing to reduce your net purchase price. Or, held in escrow to cover the expense and refund the balance to them.

Count on $10K - $15K cost. At least, that's what I paid 2 years ago. And drain field will be in another area away from the failed one. Might be a real mess, after they completely dig up the yard. Is there even room for another drain field on the property? If it's a suburban-sized lot, it's more than a problem.

Yes, I agree w/Linda117117, wait and see just what a "shot" drain field means. It could be a simple fix like needing the lines flushed. But I would ask the owner to get multiple opinions to verify what the problem is.