Let me explain.
Some businesses are profitable. Some are extremely profitable (long-time used casinos, official (those who appear in SP) strip clubs and clothing shops, ammunations. Some are not so profitable.
As the owner who had a profitable business wants to get a compensation for his lack of business - he makes a buyout. It's his compensation.
If you are not happy - there's always a possibility to buy an unowned business (it's how I bought ALL bizzes exept my first one).
About houses. Some houses ae valuable - good location, good exterior or interior. Others are ugly or far away in Whetstone, or without a parking lot. So the owner can set an extra price if the house has some benefits. (I would sell my Pershing app with a 3x-5x buyout where x is it's price). For example, a unique house at Red County (this near the river) is not very expensive, but hte buyout will be high - as there is place for any car or helicopter, and a place for a personal boat. But a crib in Las Colinas may be left unowned, if the owner find a better unowned house.
About kind of money. I don't know where from some citizens have large money amounts (the max is Melvin Gambetti with 31 million). But earning enough money for byuing a house with a car is easy - my neighbour, Lex, took me 20k, which he returned in three days.. So - in two or three weeks it is possible to earn enough cash for an unowned business.