Paris is as secure as can be, in regards to the terrorist attacks. Police and even the army are working on it, but due to the nature of the attackers (more often than not, they are youngsters from here with their brains washed...), nobody can be sure where the next strike will occur.
So to be perfectly safe, everyone just have to apply a basic safety logic, a logic that the people in Ireland, in Corsica and in the Basque Country have known for decades.
Like, if you see a car with no numberplate, or with the trunk or a door not-quite-closed... You get calmly away and you call the police.
I don't see people talking about it, but a point worth mentioning about Paris' safety rating, is the fact that we have many pickpockets, both the "silent and unseen" variety and of the "three muscular guys pushing you around" variety. Hey, not necessarily more than elsewhere, of course, but in the "City Of Lights" (TM), people get blindsinded.
As for the cost of life,
* Food is ok ; even if there is of course no upper limit to the price of a meal in a gastronomic restaurant, you'll find snacks here and there
* Rent is a killer... INSIDE Paris. But get to the "Petite Couronne" (the first towns immediately close to Paris), and the price already gets down. And way down if you accept to try the suburbs, which can take you from like, 15 minutes of RER or 45 minutes of Francilien (Paris' region train). Don't believe what Parisians think : all is not dirty and dangerous outside of the city limits