Arda Guler deal is done for Real Madrid officially. It is a six-year deal. Actually he had a release clause of 17.5 million euros. But after Barcelona's entering the race first they must have wanted to increase the offer to be more attracting. 30 million euros with add-ons is a good deal but it is still a really cheap signing for Real Madrid. They must be quite contented with having a young star for a price like this.
The reason why Arda chose Real Madrid over Barcelona was the departure time. As far as I know Barcelona wanted to have him by 2024 but Arda wanted to leave this summer. Real Madrid guaranteed that to him so he chose them. Both options were incredible for him of course.
To be honest it wouldn't have been bad if he had played another season in the turkish league (and UEL) to gain experience. At 18 and with almost exclusively turkish league experience, he could fail in La Liga and especially under the pressure under Real Madrid. He's definitely talented, but you also need a strong character and be able to handle all the pressure.