There was recently a discussion regarding different pricing of the same images on through different websites at Microstock Diaries.
Many of the industry’s best have posted their viewpoints. Here is how I personally feel…

It’s a proven fact that when products or services are priced higher, some buyers think they are getting something “better” or of “higher quality”. Many times this isn’t even the case. A generic product could even be identical, only in different packaging. Is Tylenol going out of business, just because CVS sells the same thing in a generic form? It doesn’t seem like they are. The product could probably even be manufactured in the same plant. Many times, manufacturers only want you to think you’re getting less when you purchase the same product for a lesser price. In actuality, many are identical – even if the product is packaged or marketed in a different way. Paul Mitchell hair care products come to mind. The CEO gets on TV and in commercials he testifies that it isn’t genuine Paul Mitchell, if you find the product being sold in a grocery store. In fact, it is – and I know many professional hairdressers that will attest to that. Want to know the difference? About $10-20 per bottle, depending on what salon is selling it.
If a buyer wants to be choosy and shop around, they will. If they are willing to spend extra money on photos from agencies that offer better guarantees, or a different user experience, then let them. I don’t see anything unethical about that. Product pricing is the same in all markets, it’s just all about the brand label many times that determines the pricing. Even if it’s the same exact product. Designer clothes are sold at outrageous prices, and sometimes the quality is even lower than that of the cheaper department stores’ clothing lines.