The Legend-class National Security Cutter is a frigate-sized USCG cutter that is larger and displaces more than the ex-RAN Adelaide-class FFG's. Despite being ~400 tons greater in displacement, the NSC has a significantly lighter weapons loadout and less aviation facilities. It is also priced equivalent to a modern frigate, costing ~USD$700 mil. per roughly a decade ago. The Arafura-class OPV's the RAN is currently set to have in service OTOH are costing ~AUD$300 mil. per and in today's money. Approximating for inflation and currency conversions then we would be looking at ~AUD$1.3 bil. for a long-ranged, high endurance frigate-sized patrol vessel. Also, whilst a range of some 12,000 n miles is listed, it does not mention (even on the USCG fact sheet) at what speed that range is possible. I would not be surprised if the cruising speed for such a range was well below the max speed of 28 kts.
Trying to directly compare two different vessels, of significantly different sizes, displacements and costs, is going to result in an unequal comparison. It is also worth noting that how the US and USCG operates it's patrol cutter fleet is different from how the ADF and RAN operate it's patrol fleet, as well as what the areas of interest are. Also, given that the USCG alone is about two-thirds the size of the entire active duty ADF and a part of the US armed forces, that means the requirements and intended use by the US would be somewhat different from that of Australia. In point of fact, the USCG cutter class that most closely resemble the Arafura-class OPV's would probably be the Famous-class medium endurance cutters built in the 1980's. These are 1,800 ton, 82 m cutters with a crew of ~100 and aviation facilities suitable for an HH-65 Dolphin or HH-60 Jayhawk. As a side note, these cutters were built in part of patrol the then-new 200 EEZ limit.