A 10% CTR is quite high. From what we've seen on running Ads is that:
Branded terms (the company you're providing Ads services for) will receive high CTRs - around 20-40%
Emergency type phrases that deem people want to get service very quickly and don't want to look through the entire front page will receive 15 - 35% CTR. Examples, emergency plumbers, ...
Competition isn't entirely relevant as a separate factor (it does impact CPC and CTR, but you already have ideas for these).
Basic figures would be:
Minimum: (Avg monthly searches / expected CTR) * Top of page (low range)
Maximum: (Avg monthly searches / expected CTR) * Top of page (high range)
This would give you what you should expect to set as a monthly ...