I know this is an old thread and the topic has already shifted, but maybe it would be a good idea to use custom benchmarking tool or even Corona Benchmark for the price estimates :)
Option A: Renderfarm would create a simple tool to emulate rendering by utilizing CPU at 100% and taking advantage of optimization commonly used in rendering (SSE, etc..). At the end, it would spit out score, which you would then enter into a cost calculator, and it would be remembered in your renderfarm online account, and used as a basis for cost estimates. Renderfarm would use same tool to benchmark their servers, and simple performance value conversions/comparisons could be easily done.
Option B: Renderfarm would run Corona Benchmark on their servers, and note down times. User would then enter Corona Benchmark finish time into cost calculator, and relative performance and price could be estimated.
Maybe I am too naive, but so far, I don't see many issues with this approach.
