The cost of taxis would be exorbitant..probably in the order of € 100 euros plus each way.
Bus is probably the best way to make this trip.
The following is the current week-end timetable for the bus service from Lisboa Campo Grande to Obidos and return:
Lisboa Obidos Weekend Bus Service Timetable - Click to enlarge
Or click here:
http://bit.ly/lisbon_obidos_weekend
