It varies state to state, but the most popular certifications in my state are ACE and NASM, and those are the ones the big box gyms like to see. Cost varies depending upon how much studying you want to do. You can just take the tests for $100-200 or you can buy materials to help you which would run up to $500.
Local gyms don't really care what certification you have, as long as it's a nationally accredited cert. In my state, you don't even need a certification to personal train or to get PT insurance.
I personally have a nationally accredited certification, a degree in Kinesiology, and PT insurance. All that stuff doesn't really matter though. My clients don't give a shit about my qualifications though, they care about my experience and ability to get them their results safely.
Though there are some great certifications that can teach you great info with workshops and continuing education, most certifications are just pieces of paper and honestly a joke. I remember when I was originally going to get the ACE certification, I read an excerpt from the book that explained the "Talk Test." The Talk Test essentially explained that if your client cannot carry on a normal conversation with you because they are breathing heavy, they are working too hard.