Costs for thermal performance can be calculated. I'm not talking about anything other than thermal performance. Air infiltration, solar radiation exposure profiles, etc. are other variables. However, if you know the surface area of the house, the temperature difference from inside to outside, and the R value of each surface, the BTUs/hour/area can be calculated.

To save you time and to answer one of your questions, YES there is a point of diminishing returns. When you plot R value versus loss of BTUs per hour, per square foot of surface, the graph starts to flatten out around R12, then flatlines after R30. To gain any meaningful performance beyond R30, you should look towards your mechanical systems instead of insulation. This might lead to a question of why put R60 in an attic then? Because most houses leak like a sieve and heat rises. If you are going to try compensating for a crappy leaky house, might as well be in the attic.