A discount is simply a method for drawing in more customers, but never an obligation, no matter how much the other party may make it seem like to the artist (seller).
Personally, if I feel like I might want to try to see if I can get a better deal, I would try to ask. If the artist is not willing to negotiate for whatever reasons, I have exactly two choices from there:
a ) Take his current offer, or;
b ) Look for another artist (who may, or may not, produce a similar quality to the first artist for a lower price).
Name-calling, demeaning or shaming the artist is not going to get anyone far. Sure, the commissioner might get his/her way this once, but only this once. The same artist is not likely to take another commission from this person. This person could try the same tactic on other artists, but he/she is going to run out of avenues very, very fast.
But let's be serious here, the artists available in RMW are amazing and talented, but charge very low prices for their work (especially if people consider the hours of work that went into them). Speaking objectively, I see little sense in asking for an offer on the current low prices for their good work.
Wait for the artist to offer a discount, if a discount really must be had. A supermarket would sell items at a discount only when dictated, or under special circumstances, not when a customer throws a tantrum (ie. just because that box of chocolate is $1 more than he is willing to pay for it).
Let's not forget the famous, modern day, rather stereotyped ios/android users who want high quality apps, yet insist everything on the market should be not more than $0.99, and even then refused to buy them at that price, stating that they should be 'free'.
So, nio, if you feel your prices are reasonable, and there is no reason to give a discount, don't. Definitely work out all prices and payment method before agreeing to a commission, and stick to it. If you want to throw in one or two free artworks after that because that person is a good client, then that is up to you.