The cost of living in Lisbon has been going up in recent years. Blanca/Valbuena’s Guide to how much you’ll pay for things in Lisbon is a great resource to start out with..
Public transport is very affordable – 36 Euros for a full month, unless you’re based out of zone 1, then it’s a little more. In particular, buses are a fairly inexpensive way to get around both Lisbon and Portugal. For inter-city busses, you can as the station assistants by the Zoo for help or you can research specific routes online.
There’s also sites like CarpoolWorld where you can share a ride somewhere. The only caveat is that if you need a return ride, it could prove problematic.
There is a cheap, Chinese-owned shop on rua poco dos negros not far from the tram station. They sell everything! There are a few independent stores around the area that sell affordable kitchenware and much more, you just have to wander around a bit.
Food is relatively affordable (in line with UK prices). A cheap place to eat out is the RC-Restaurante on Santa Apolonia. Five Euros for good food which includes a 1-2 L bottle of wine or beer, a Coffee, and dessert.
Renting out a room is way cheaper than have a place to yourself. The average rent for a small apartment would be minimum. 650 Euros. Cheaper places are possible to find but most are far from the centre. OLX is a good website to find rentals direct from owners, but the site is also full of rental-agent listings so you need to look several times daily as there is a lot of competition on the demand side.