Property records for the county show that Koch bought his first Aspen house for $1.9 million in 1989. Two years later, in 1991, he paid $1.75 million for another home on the same street.
Today, the first home has an estimated value of almost $18 million, and the second is valued at $8.6 million, according to Zillow.
The Kochs also had a 30,000-square-foot mansion in Palm Beach, Florida, called the Villa el Sarmiento, that they reportedly bought for $10.5 million in 1998 and spent another $12 million renovating.
Today, the mansion is worth an estimated $37 million.
A representative for Koch Industries declined to comment on the couple's real-estate holdings.