I've already tried getting into this and I thought it's worth calling ahead before you make a decision. The gyms I know in Lisbon have pretty different pricing - some have free sign-up promotions if you lock in an annual contract right away, others charge an entry fee between 20 to 50 euros. The monthly fee varies because some only offer group classes, while others let you do extra sparring or individual training with the coach. In Parque das Nações you've got pricier and cheaper options depending on the facility.
What I think is that the range people mentioned before (60 to 120) is a bit too broad - there are more affordable gyms that cost less, that's true. But honestly if you want to know for sure what it'll cost you, you have to contact the gym you're interested in and ask: whether it's month-to-month or annual, if there are hidden fees, if you can do trial classes. Some let you come once for free just to see if you like it. It's always worth comparing half a dozen before you decide.