That's exactly what our September flights have cost with Jet2.com.
Ryanair (:shock:) fly to Porto from Liverpool and,if you book early enough and are flexible with your dates,the return fare for two people can be as little as £65.The airport at Porto is not too far from the city centre, so transfers are not expensive and, from what I read, as long as you choose your hotel carefully, everything is within walking distance.
The only forseeable problem is that Ryanair are really strict as regards carrying excess weight and all the Port Lodges are in Porto...............