Hidden costs buyers only discover after possession!

Currently in a 2BHK in Sector 75. Thinking of upgrading, but the hidden charges are scary. Heard Jaypee adds a lot during possession. Supertech's RERA compliance is questionable, right? ATS Green is better, but is the price jump worth it? Anyone faced issues with these builders? Already have a home loan, don't want extra surprises. Any tips?