Just for comparison The Swan and Dolphin are currently undergoing a $75 Million renovation. If the two buildings are undergoing that much renovation, why would $300 million be out of line for all of Future World.
From a construction point of view, it is always cost more to rehab/renovate than to build new. Take a look at the cost breakdowns for "This Old House". I recently saw a repeat of a show from a year or so ago in which a 1-1/2 story house was stripped of paint, wood filler applied to holes, etc. and painted. The cost for re-painting this house...$57,000!
Also, the cost for building the next "country" in the World Showcase is estimated to be at least $50 million for a single pavillion. Etc., etc....