You'll have to have a pretty relaxed attitude to get around by cab in Nosara and not be shy about calling for a pickup because they aren't parked on every corner. We've done it and if you don't mind walking shorter distances (a mile or so) you'll be alright. If you're going to be out very late you probably want to make arrangements for a pickup time and place in advance and make sure it's clear you're a big tipper.

Nosara/Pelada/Guiones/Garza is spread out and it can be really nice to have the mobility of a 4x4 especially if you want to go farther afield to Ostional or Samara for example. Rentals are very expensive in Costa Rica and although $450 a week is a little steep I wouldn't expect much less than $350 a week for a little one once all the insurance etc. is added in.

The airport transfer is about right. It works out well if there are six or eight people sharing but gets pricy for one or two.