Was just watching the episode when Barney was informed that his apartment he lived in was turning condo.

Probably despite the 14% interest rate at the time it would have proven to be a good investment, I think the episode aired in 1980. He could have later refinanced.

The cost was 137K, presuming it was a 3 bedroom since he had a boy and a girl, it would probably be in the 2-2.5 million range today.