Yes there really is no set way to determine the market value for rents.
Here in Minnesota, what we generally do is take a look at the competitive houses, condos, townhomes for rent and price accordingly. You should be able to find a number of comps in the newspaper as well as online.
Generally I like to figure a certain dollar amount per bedroom. Here in the Minneapolis real estate market, you are looking around $400 a bedroom (approximately.) I hope this helps!