Capital Gains on House Sale

My parents were given land from their parents over 30 years ago. They build a house on it and have lived in it all this time. So, they have just the price of the house put into it.

At some point, they got a mortgage on the house and have carried one until today, now owing around $200k on it.

If I bought the house from them, paying them just what they currently owe on their mortgage, would they still owe capital gains on the amount between what they paid to build the house and what the mortgage balance was (probably about $150,000) or would they owe no capital gains because they did not realize any profit?

I know they could usually get an exclusion, but it would only be half because half of the property is used for a business (they receive rental income from a separate business that uses the outbuildings).

So, I'm thinking they would owe capital gains on half of the difference (200k for house sale - 50k to build house / 2 = $75000). Am I right that they would owe around $11k in capital gains if at the 15% tax bracket for capital gains?