Hi,
I'm a pilot for a portuguese airline and I can give you some tips about the cost of living in Lisboa.
To start with it, it's better to introduce you all to one of the most violent V.A.T. in Europe: 23%! Almost everything you buy (clothes, electronics, furniture, even the restaurant bill!!!) is subject to this crazy tax!
You'll hardly get a decent 2 bedroom house for less than 950/month.
Although the Airport is in the city, the underground network is completely annoying! Only if you live in Parque das Naηυes (former Expo 98) it can be valuable for you. Otherwise you'll have a big underground journey ahead everytime you go to work!
If you intend to buy a car, do it! BUT FAR AWAY FROM HERE! If you are an EU citizen, bring your own car, otherwise you'll pay for a car as you never did before.
So, I don't think Lisbon is that cheap!
Dine Out in a good (I'm not talking about top restaurants) restaurant for 2: 80/90. (Complete meal, without Wine)
McDonnald's average Menu: 5,50
Underground ticket: 1,40 (valid for 1 hour) 6 (valid for 24 hours)
Gas: 1,65/l Diesel: 1,45/l always changing! (for worse)
Private School: 500-1000/month
Electricity: 70-80/month
Hope it helps!
Good luck!