Hi, unfortunately, it's really hard to price her since there haven't been recent sales. The most recent sale I have found of her was on eBay in April 2016 & she was paired with a prototype white celestia. The listing says it ended for $250, but I can't say for sure that it sold. The winged white celestia prototype seemed to be more highly sought after & valued higher than celestia/rarity.
If you wanted to sell her you might want to take offers. I would guess that she would be valued in the 75-100 range. She may even go higher to the right buyer, but again it's hard to know since there haven't been many up for sale.
I would call her a White Celestia/ Rarity Prototype. (I believe her head is still different from the white fashion style Celestia Hasbro eventually released or at least the hair is different so she is still relevant as a prototype).
I know that's probably not the most clear cut answer, but I hope it helps. She is a beautiful pony, I have one & absolutely love her colors.