How to figure market rents?

[Sean Dezoysa]

I live in CA where MH parks are around, but not very popular (about 4.5% of total residential housing). Finding the lot rents are easy enough, but how do I find what the unit itself would rent for? (I plan on buying units and selling or lease/optioning units).

I've heard that you can look to local lower end apartments and subtract 20% but that seems like a rule of thumb at best.