The Yellow Cab company costs 45 (quarente cinco) soles from the airport to the Westin. the guys are right there when you exit from baggage claim. I'm pretty sure the prices are fixed, that all rides to San Isidro are the same price. They have fairly new Toyota Yaris

Once you are here the concierge will be more than happy to call you a cab - its something like 55555, and its cheaper than the yellow cab, except you have to negotiate the price before you get in (but the concierge, especially Giancarlo, will be glad to tell you what it should cost)