Never, NEVER, try to "negotiate" a "price" with a provider.
You could quite possibly be one the most idiotic men alive and deserve the response you will get.
What kind of moron wants to start a transaction off with a woman who will hopefully provide an amazing intimate experience by saying "You're not so great, You aren't worth what you think you are". Try that type of attitude as a pick up line out socially.
"Hey Chevy, I really like the truck I'm thinking of buying but Chevy really sucks and I don't want to have to pay for it"
That being said, I don't believe it would be wrong to send a message in the proposed scenario, THAT DAY, saying "I saw you are offering a special, and can't make it today, would you be willing to see me on "X" for the same? If not I apologize for bothering you".
I don't know this guy, I don't delve into any particular clowns posting history on here, but I do have respect for Kram and OldRanger and will go with their comments on the original poster.
Back to rule number #1
DO NOT TRY TO BARGAIN WITH A PROVIDER!
If she, or her services, do not fit your financial means or criteria then just say "Oh, well, I guess I won't see her", move on and find what may.
You make life more difficult for the ladies many of us want to see, and for those of us who want to see those ladies, by wasting their time, annoying them, and then making it harder for the rest of us mongers because the lady in question has to be more guarded because of your time wasting and insulting antic's.