I'm no expert on the subject, but when we bought our RX 330, we opted to get the Lexus factory extended-service contract (I think they call it Platinum Premier coverage) that gives us 6 years/100K miles. The Lexus "sticker prices" for an RX 330 (and also for RX 300) are:
6 years/70,000 miles: $1775 with no deductible, $1725 with $50 deductible
6 years/100,000 miles: $1825 with no deductible, $1775 with $50 deductible
7 years/100,000 miles: $2150 with no deductible, $2100 with $50 deductible
When we were dealing with the finance manager, she said that the cost for us to get a 6 years/100,000 miles with no deductible would be $1160.
I'm not sure if the dealer with whom you spoke was directing you to a 3rd party provider because you're in a vehicle that has been on the road for a while, while we were looking at getting one at time of sale, but I had been led to believe that you could buy a Lexus extended-service contract from any Lexus dealer at any point before your eligibility expires. You might want to go back and question them, and/or contact another Lexus dealer to compare pricing. I believe the consensus from automotive writers is that a factory-based program is preferable to a 3rd-party program.
Good luck.
Dave