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"To me it seems like the play is just taking the bloated insurance at the counter? Any other ideas?"

Yes: as suggested earlier, investigate the cost of a non-owned automobile policy. Mine is issued through Travelers Insurance, and my most recent premium for six months' coverage was $29. That gives me coverage of $500,000 for third-party personal injury, and $100,000 of coverage for property damage. And because I am a New York resident, the policy automatically includes rental-car loss/damage (paid out of the $100,000 property-damage coverage).

Does your MasterCard include rental-car coverage? If so, contact your card issuer's insurance administrator, and ask whether car rentals that are prepaid via Priceline are covered. If they say "Yes," ask for them to confirm that by E-mail, so that you have it in writing.