You should be able to get it for £310 from bookings.com. So far everyone is staying there (it's got a pool/ spa, and is highly rated etc.) But there are other places nearby that are cheaper.
If you go to bookings.com, and choose Porto, the Boavista area, you'll find that well rated cheaper hotels include the Sheraton (£266), The Bessa (£177), or the Quality Inn (£166)
Hope this helps.
Or perhaps someone will suggest sharing?