Quick Google for the Census Bureau only turns up median rather than mean:

https://www2.census.gov/programs-surveys/decennial/tables/time-series/coh-grossrents/grossrents-unadj.txt

Median is probably a better value here since it’s going to reduce outlier effects.

Looks to me that median rent in most states in 1990 was closer to $300-400 per month than $500.