With the economy in the state that it is already i think this would be a terrible idea. We already have people with >$1 Million. Meaning no matter what we set the price to they can just buy several from the start where as new players will have to save money for a long time.
I also think this would cause more lag problems for the server due to people making elaborate mob farms that use multiple spawners.
Going to go ahead and say this is highly likely not going to happen, it's just got too many problems with it.