Rack-rent denotes two different concepts: The second definition is equivalent to the economic rent of the land plus interest on capital improvements plus depreciation and maintenance -- the normal market rent of a property—and is not inherently excessive or extortionate. Also, this may be different from the rent actually being received. Historic...
• (n.) A rent of the full annual value of the tenement, or near it; an excessive or unreasonably high rent. • (v. t.) To subject to rack-rent, as a farm or tenant.
A market rent usually only revised upwards.
Full market rent. Contrast Ground rent.
Rack'-rent` (-rĕnt`) noun A rent of the full annual value of the tenement, or near it; an excessive or unreasonably high rent. Blackstone. Rack'-rent` transitive verb To subject to rack-rent, as a farm or tenant. 
[n] - an extortionate rent
noun an extortionate rent