I hate to be obtuse but there's a couple factors that determine how much I get for estimating. Some of the other members could probably help in this regard much better than me
There are 2 commercial contractors I do estimating for. In most cases I actually go to the site (when required) meet with the representatives, go over the plans, ask the questions, make the calls, and then run the numbers. In those cases I am on the clock and have an hourly figure ,$--.00/hr
In a strict case of simply looking at plans, doing a take-off and then crunching some numbers you will probably have to look at some type of graduated system based on job size. I just did a 20k interior commercial remodel estimate that took about 8 hours due to numerous clarifications. It was a fairly complex project. Other 20k jobs might take me 2-4 hours.
A system somewhere around 1% with a bonus for landing the job would probably work - run the numbers and see. It obviously depends upon your region, competition, and profit margins.
Hope this helps. If i can be of any help drop me a private email. I do consulting on a number of areas including safety and management.