Challenger Ahri NOT really on sale!!!???

I did the update and saw the challenger ahri skin was now on sale. I Had a little over 1000 RP left, and at 487 RP for a good skin that has limited availability it sounded like a good deal! But after I clicked purchase it said i had 50 RP remaining.. and after looking to see how much it charged me it said 487. But I definitely know without a doubt I had over 1000 RP. Can someone please correct this please?