@Carl wrote:
The best definition I can come up with is this.
You do not have to guess at this nor formulate a definition since it is set forth in an IRS publication:
Fair rental price.
A fair rental price for your property is generally the amount of rent that a person who isn’t related to you would be willing to pay. The rent you charge isn’t a fair rental price if it is substantially less than the rents charged for other properties that are similar to your property in your area.
See Publication 527 (2020), Residential Rental Property | Internal Revenue Service (irs.gov)
[The passage in italics above is also in the TurboTax guidance]
