How much does busy street noise lower an apt's rent/mo?

[Andy Kaye]

I'm trying to price the rents for my apartment building right now. It is on a busy street, but the lot is long and skinny so the units closer to the back of the lot are quiet while the front unit is very noisy. 

All my units are 1br/1ba, and I've successfully rented out one of the quiet units for $1250/mo. How much should I expect to get for the noisy front unit?