I'm a prime member, have been for a while. I just tried to import my library, it said I had 250 songs I could import for free. It then asked me if I'd like to pay $25 to upgrade so I can import any number of songs. I was able to upload all my songs to google for free.
Dang. Didn't realize that Amazon charged you past a certain number of songs. I have my whole library uploaded to Google as well. I usually use spotify, but it comes in handy sometimes (like when I want to listen to the Beatles).