I found this link and it reminds me of your new property build out. Crazy install prices!
The cable company is not ripping anyone off. Those people just bought a house outside of the broad brand distribution. They should’ve done their research before assuming that they would be able to get cable service. I see this happen way too often. How do I know? Because I work for Comcast Cable
We also don’t know if the prior owner fought to keep comcast from burying any lines to the property and it is something buyers should be researching prior to purchase. Given that the problem is in cost of burying a line I would see about getting my own quote for that part.
Comcast covers the cost of a typical bury drop replaced or new run. However if the home is placed in an unusual location not intended for cable service or is outside of the designated service zone the cost could be passed down to the customer if they are requesting broadband. And in most cases those same customers want it done in a very custom way, which always forces a charge