I'm not trying to be difficult, but, under what standards are you able to determine how much a theme park attractions should cost. Are you in the business? Have you been involved with building attractions and therefore know what a reasonable price is to do them?
Normally, and I don't pretend to know how Disney does this, when a project is being started, bid sheets are sent out to contractors, completely detailed with itemized needs, from earth work to rough construction to fine details. The qualified contractors are notified of the bid and do quotes to the requesting company. Each bid from each contractor is opened, usually in public to keep it honest, and the LOWEST bid is awarded as long as they qualify to do the work.
If a contractor qualifies and has the low bid, they get the job. In the event that a bid is lower, but, the bidder is not qualified then it can get awarded to the next lowest bid. In other words Disney cannot always control what the cost is. What they can do is cancel the project and start over, but, then that much time has been wasted and the process starts over. Since for most of Disney's projects the government cannot insist that bids have to be asked for since it is private money, however, stockholders may object to uncontrolled spending.