During our stay in Estoril, we went in order to see MotoGP, we chose bungalows in a campsite called Costa da Caparica, about 20 minutes from Lisbon (that is without traffic) and they are really affordable, I remember it cost us about 15 euros per night per person. The bungalows are usually for 5 people, 3 single beds and one double, and truthfully the mattress is very comfortable- they are those which have no springs and let you fall asleep really easily. I will also comment that it is a very quiet area and they have car parking right next to the cottage.
Located in the centre and on the main street Mar e Sol is the solution for those who want to have a vacation at the beach, and wish to be two steps away from the capital (Lisbon). This accommodation was refurbished in 2004 and the rooms are very economical.