Simple for me, if I fancy her I'll still punt her.
However, there are a few extra steps to consider:
1) do you fancy her as much as the girl you wanted to see? How much would you have been willing to spend if it had been this girl's profile you'd seen on AW? That is your new rate. She accepts it or you walk.
2) regardless, you now have the power. Point out that you know it's a bait and switch and that you'll walk if you don't get a discount. You may even fancy this girl more, but one way or another you should be leaving this punt with your wallet heavier than expected.
3) it is always important to check services on the day before handing over the cash, but is now of paramount importance. This is a different person with different likes and dislikes.
4) no matter what happens, this is a negative review. The switch might be Scarlett Johansson, the sex amazing, and the rate reduced to a tenner, but a bait and switch is an automatic negative.