I like the idea, but I think it could be 10 HBD for less than two months (or one month). Then the cost immediately rises to 100 HBD for more. And that's it.
We need some speculative proposals. If it will cost 50 HBD for a two month proposal, then no one will dare make a proposal unless they are already a witness/ 100% sure to be approved.
It's important to allow anyone to create a proposal cheaply that won't only last 10 days. Two month seems a good compromise to leave a proposal there long enough while keeping the DAO clean.