Obviously, in a properly optimized campaign, the more you spend, the lower the overall CTR (in theory).
Your Brand should have a much higher CTR than generics (and cost much less per click). I recommend setting separate benchmarks for brand terms vs others.
Your vertical will also change the benchmark.
However... Google has some tools to help you predict traffic levels. I'll give a suggested benchmark for you... but everyone after is likely to say "you can't predict a bencmark without knowing all the facts". They are right. But strive for a CTR of 10% on brand and 5% on generic.
Then strive for 5% conversion on brand and 2% conversion on generic.
I don't know if those numbers are right... but hey, you have to start somewhere :)