You might find this post from Hive account@ilikechocolate interesting.
It looks at the idea of normalizing BOINC credit based on estimated power usage per operation (FLOP), there are some calculations for common processors and graphics adapters.
You could probably use the figures together with statistics form a State of the Network post to compare cost per 1K Recent Average Credit with the Gridcoin reward value per 1K Recent Average Credit.
You would have to do the comparison for each project separately and re-calculate as new statistics become available.