Enough people buy $100,000-plus houses these days to make ordinary mortals wonder who those people are and where they get the money.

Now it can be told. They are mostly ordinary people, married couples in their 30s, both working and earning combined incomes of $38,000 to $39,000 a year. Their special characteristic is that they owned another house to sell or rent to plunk down the money for a $100,000 nest, according to a survey of Walker & Lee, an Irvine (Calif.) based residential real estate company.

Walker & Lee reports that the median price of houses the company sells in Orange County, Calif., is $113,000. In 1977, the firm sold $16,070 new and used houses, mostly four-bedroom models with an average of 2,322 square feet of living space in two-story styles.

The firm itself wondered who could afford such houses, said George Fulton, senior vice president for corporate marketing, so the company surveyed buyers of 212 houses price between $100,000 and $120,000. It found:

The average buyer was 38 1/2 years old, although a few were younger than 25 or older than 50.

Ninety percent of buyers were married.