Try it without a membership first - go to a friend's gym for a couple of weeks or use their free trial week. That's the best way to find out if you'll actually stick with it or if you're just kidding yourself. Sure, 10-15€ a month isn't expensive, but it's still annoying to throw money away if you stop going after two weeks.
Here's the thing: it's less about the price point and more about whether you actually make it a real routine. Some people need a place with equipment and an environment to stay consistent. Others prefer training at home or outside and would just be wasting money. If you notice that you really won't work out without a gym and the cost is peanuts for you, then get a membership. McFit and Fitx are cheap enough that you won't lose much if it doesn't work out.
But what does help: set specific days and times beforehand for when you want to go. Not "sometime during the week," but Monday and Thursday at 5 p.m. or something like that. And give yourself at least 4-6 weeks before you give up. The initial excitement always fades, but then it becomes a habit.