There isn't 4500 who walk to Pittodrie. I'm happy for you to go with the percentage you pulled from the survey for the sake of an argument but that survey was available to all Dons fans, not just season ticket holders or folk who have purchased tickets through the club.
I don't expect 4500 to walk to Kingsford. I expect loads to take advantage of the onsite parking, shuttle buses and park and ride facilities though.
Using an inflation calculator, in todays money that's £30mish. The training facilities will cost £10m. So there's a ten million pound difference. No idea what state the location was in Middlesbrough before the stadium was built but there is significant to be down on site at the moment.