We stumbled upon this attraction and boy were we glad to have done so.
The entrance fee was just a few euros (4 or 5 pp, I think) and we could have spent longer than the 2 hours we did. Beautiful displays of bishops robes, ornaments and other church regalia are combined with excellent cloisters and other masonary feats. There's a long history of the Portugese royal family's association with many being buried here. Take the long climb to the roof top where you can lose at least half an hour gazing at the fantastic views of Lisbon and snapping photos that will impress! On a sunny day this is a superb experience. You can take the No.28 tram almost to the front door, or walk over in about 10-15 minutes if you're visiting the Sao Jorge Castle nearby. While we saw a number of ramps and facilities those who have trouble with stairs are going to miss out on the Lisbon views.