Are aerodynamic rims worth additional weight?
Obviously the prices seem to support the idea that aerodynamics matter
  more than a few hundred grams of weight, but at what point does the
  added weight offset the gain?


An exact calculation will depend on the total mass of you and your bike, your speed, the wind, its angle, whether you're climbing, on the flat, or descending, and the speed you're going (or the power you're putting out). However, we can make ballpark estimates of many of those variables to get an approximate answer. 

Let's say the aero wheels add 100 grams of total mass compared to the "stock" wheels, and in return reduce your drag area, CdA, by .005 m^2. That's a ballpark improvement for a reasonably aero wheel compared to a standard box rim, though for extremely well-designed wheels you can see perhaps double that difference (~ .01 m^2) or more, especially at high yaw angles. 

The power equation for a bicycle is well-understood and was given in this Bicycles Stackexchange answer. Thus to determine the point at which it is preferable to trade off lower weight for aerodynamic drag, we can substitute suitable values for mass, speed, slope, and so on, and plot the power savings, as is done here. 

The figure below compares an 80 kg rider plus bike with standard box rims vs
an 80.1 kg rider plus bike with aero rims. We presume the aero rims save .005 m^2 in drag area compared to the standard rims. Three dotted lines show the power savings for a climb of 5%, a flat road, and a 5% descent. The x-axis shows the speed of the rider in km/h, while the y-axis shows the savings for the lighter wheels -- when the dotted line is above the solid horizontal zero line, it is better to have lighter wheels; when the dotted line falls below the solid horizontal zero line, it is better to have the more aerodynamic wheels. 



As you can see, only for steep climbs at low speed is it preferable to have lighter wheels; however, for this particular comparison of weight savings and aero drag savings, the benefit is small, less than a watt. As speed increases, the dotted line eventually dips below zero and it becomes better to opt for the aero savings. 

That was for a steep hill. On the flat and for descents, you will almost always be better off with the more aero wheels. 

Note that the savings in power is still relatively modest. When racing, even small benefits can accrue to determine victory or loss, but for normal recreational riding you may want to keep the magnitude of lighter wheels vs more aero wheels in mind, especially if your budget is limited. Only you can decide whether the relative benefit is cost effective.