To find the median value of the houses, we first need to arrange the values in ascending order:
150,000, 150,000, 220,000, 220,000, 220,000, 220,000, 490,000, 490,000, 490,000, 540,000, 540,000, 800,000, 800,000, 800,000, 800,000, 800,000, 975,000, 975,000
There are 18 values in total, so the median will be the middle value when they are arranged in ascending order. In this case, the median value is the 9th value:
490,000
Therefore, the median value of the houses on Mango Street is $490,000.