According to an individual in Monticello (I found this on the person's blog), two families donated 200 acres to the foundation; I'm not sure where the foundation came up with the remaining 320, but at least some of it was purchased:
"Shannon[sic] Brooks from George Wythe College and his wife stopped in to check out the store. They think they might know a few families coming with the college who would be interested in buying it. We will just wait to see what all happens.
"It looks like [redacted] and [redacted] are going to donate 100 acres to the school and [redacted] and I are donating 100 acres, and the college will negotiate with [redacted] and [redacted] for the other 200 they need. We are buying 100 acres back, and then donate that to use as a tax deduction. ...We will be fencing this summer and maybe start on the pavilion."
All of this land was later annexed by the City of Monticello. You can read the annexation document here.