There is a lot of ways how to go from Porto to Lisbon.
You can use train, bus or plane.
For us was the cheapest plane with RyanAir.
You have to go by plane from Porto – Francisco Sá Carneiro Airport
Your final destination is Lisbon – Lisbon Portela Airport.
Single: 9,99 Euro
Return: 19,98 Euro
Duration: 1:00 h
(Attention: these prices vary, € 9,99 is the lowest price. You have to find a suitable date and every day the prices change)
Timetable – schedule
You can find very cheap accommodation on Airbnb.com
On this portal are a lot offers for accommodation, if you use this registration promo code, you have 20 Euro discount for first use. – www.airbnb.cz/c/ondrav1
Another alternative is cheap hotel in Lisbon. (With this link you will have 10% discount.)
Perfect is go there at friday morning (6:30) and go back at sunday evening (20:25). So you spend less money for only two nights through cheap Airbnb.