Can't help you with Sea World, but we took taxis last week from Pop to and from the Poly and Grand Floridian for early breakfasts. Cost was about $18 each way before the tip. I would guess MK would be somewhere close to that as well.
But, keep in mind, a taxi to the MK will drop you at the TTC and you will then have to take either the boat or monorail over to the MK. The bus is much easier and quicker.