What are they including in the Sintra trip? Here are the options:
http://www.parquesdesintra.pt/
The train ride is <4€ each way. The bus from Sintra to Cabo da Roca and Cabo da Roca and Cascais is also similarly priced (it runs only 1/hr). The tour will probably be 1.5x more as expensive as if you travel on your own. The question here is how much time you have to explore. The three can easily be done in a day without a guided tour for ~ <30€/person (rounding up everything, not including meals).