Base only on what the OP posted, in no way do I see this as inulting. It was a very polite way of saying "I would love to see you but for some reason I cannot at your rate right now. If you should happen to offer a lower rate at some point, for what ever reason, I woud be interested." That does not mean he is saying your not worth what you post or any thing like that. Of course, one of the factors we dont know is what was he willing to pay? If you asking $300 but the guy is willing to pay $250, then some, even many, women, depending on a number of factors, may be willing to accept the slightly lower rate.
Why would it be insulting for someone to politly ask if you are willing to accept a different rate? If is not intended to devalue you or say your not worth what your asking. And as someone else said, if it did not work some of the time, guys would not do it.
If you are not willing to consider a lower rate, then good for you. I see no problem with that at all. In my business, I also charge what I feel my services are worth and have a minimum I will not go below. If you want cheap, go somewhere else. If you want good, then I am your man. :-) Either do not respond or a polite thank you is in order. You do not want to burn your bridges as you never know what may change tomorrow.
All that said, I have never asked for a lower rate and would not do so with a provider. But it is amazing how many times I have had providers who I have met before having a session offer me a discount or special deal of some kind. I have had some offer me as much as 1/2 off or very extended time frames. Go figure. 20+ years of experience tells me many, if not most, are willing to negotate but just don't want it known.
And Sam? You rate is too low!! But don't change it till I have a chancve to see you!
Strippers? Hell yes. But that is a different industry and its far more expected there.
Now, insulting would be "Hay bitch. No way your pussy is worth 400. I got 150. Want to fuck?