What about this for an idea...
If, for example, the black mamba shouldn't cost $250, how much SHOULD it cost? $1000? And how much SHOULD a normal corn cost? $200?
If all the breeders out there decided to sell their cb animals for more, wouldn't that just encourage poachers to capture MORE animals because of the higher profit they can snag? I think this mentality would carry over to any regularly wild caught species.
The prices have likely settled to an amount people are willing to pay given each species' availability, aka supply and demand. The low price of certain species likely contributes to their popularity. For example; if a newbie liked both the Honduran Milksnake and Normal Cornsnakes equally, which do you think he'd choose as his first snake, one that costs $30 or $175? And that's partly why some of these cheaper species are so popular, especially with beginners.
Now, I agree with the "Disposable pet" syndrome, but what can actually be done to change it? Would anybody pay $300 for a leopard gecko or $200 for a normal cornsnake when they could get one for far less?
*Note: All the figures used are just for argument's sake