For me the taxi rate seems reasonable as well, for me the rental sounds like a great deal.
If it is with EasyCar, they have some great deals. My parents rented a car in Santiago with them, and got a very low price (don't remember how much it was). They are owned by the same group that own EasyJet (Low cost airline), so I am assuming they have the same Low-Cost ideology with EasyCar.
Note that EasyCar does not have a single office in Santiago, they use the excess capacity of other car rental companies (I think they use the cars from Atesa Car Rental in Santiago). So when you book with EasyCar, you pick it up at a Atesa office (there is one by the train station in Santiago). We had only good things to say about EasyCar (and Atesa) when I helped my parents with their car.
Good luck Sil, would be great to have a coffee with you, your friend and your husband once you get here.
Un saludo,
Ivar