I used to do work in commercial real estate valuation, and in that world, this is a very basic question.
What you want to find out is the cap rate of comparable properties. The cap rate is the direct link between the value of an investment property and the (annual) income it produces.
Value * cap_rate = annual income
You have the first value, can research the second, and want to know the third.
Obviously this is a simple calculation, and the real work is in determining what a proper cap rate for that property is. This is part of why you pay hundreds of dollars for a professional real estate appraiser to produce a valuation report. Appraisers will use certain services to scan databases of real transactions to find comparable properties. Some places that provide real estate comps actually list the cap rate for specific properties in their databases. One such service that I've used in the past is
http://www.costar.com/Products/COMPS/. Unfortunately, these sites aren't free.
The cap rate is determined by factors including the local real estate environment at the current time, the location, and the "class" of the property (how nice it is). Without any further information regarding the property in question, there is no way to tell what the cap rate would be. There are people out there who put out periodic (usually quarterly) reports on the state of real estate in major markets, broken down by alls sorts of stuff, and tracking trends over the last few quarters. Cap rates will almost certainly be in there.
It is likely that you can get cap rate information from free sources, but I've never done it.
Based on my admittedly out-of-date knowledge, I'd say the cap rate would be anywhere from 5-6% for a very desirable property to 20% for one that isn't. Based on this range, a $1.2M property would yield an annual income of $60K to $240K, or $5K to 20K per month.
edit: I'm curious, what is the market, what sort of property is this, and what is your interest in knowing (for example, for tax/estate purposes, an appraiser will not go exclusively by cap rate to justify a lower appraised value, and thus a more favorable taxed amount for the client)