...............how much downforce would an elevator produce on a conventional, aerodynamically stable aircraft in unaccellerated level flight,..............
This is not easy to answer. Calculating the force on a tail is a complicated matter. For one thing, elevators (I assume you mean horizontal stabilizers used in conjunction with elevators) don't always produce downforce in spite of what you may read on the Internet.
It may be important to mention that not all aerodynamics experts fly aircraft, but everyone who can fly considers himself an aerodynamics expert.
When you look at conventional aircraft (non-canard) and consider the combination of the horizontal stabilizer and elevator you can find examples of tails which produce downforce as well as tails which produce lift.
Most aircraft have positive longitudinal stability and generally speaking, this feature is desirable; very desirable. This stability can be achieved with a tail providing downforce, providing lift, or in the case of flying wings, no tail at all. Or, given a sophisticated enough flight control system, an unstable aircraft can fly just fine too.
Stability and control (they're two different things) go hand-in-hand. This discipline can be a rather complicated and specialized area of aerodynamic study. Furthermore, some of the laws of aerodynamics aren't easy to understand. When it comes to the most simple explanation of longitudinal stability, using an example of a tail producing downforce makes that explanation easier to understand.
Those who study aerodynamics in a serious way nearly always refer to something called the Aerodynamic Center; Google it. The aerodynamic center is the point at which the pitching moment coefficient for the airfoil does not vary with lift coefficient. Wrapping your head around this concept and what it means takes some thought; perhaps a lot of thought. Especially when you realize that it involves the pitching moment COEFFICIENT, not the pitching moment itself. Google is your friend here. Try searching on pitching moment, aerodynamic center, tail downforce, aircraft stability, etc. You'll need to understand the AC before you get too deep into stability.
Of course, stability is not the only factor that must be considered. For example, an arrow with a heavy arrow-head is stable, but it will fly in an increasingly nose down attitude. Aircraft must be not only be stable but must be able to be trimmed too, as we all know.
By selecting the right airfoils (usually this means a horizontal tail with a different lift slope from the wing), the correct plan-forms, the correct CG, and the correct incidence of the wing and tail, one can design a stable aircraft which can also be trimmed while at the same time producing positive lift from the tail. This idea isn't nearly as easy to grasp as the chalk drawing dealing with stability which we all had shown to us before we got very far in flight training. But if you dig deep enough you'll find the answers you're looking for on the Internet.