If you are creating a position, I have not done that.
If you are proposing work to someone in hopes that they will buy it.
1) Make sure that you give a clear definition of what it is you are porposing to do. In that way if there are issues after you are finished the original document will serve as the checkout form.
2) Try to get a handle on how much the traffic will bear on price. Too high a price and you will be rejected out of hand. Too low usually means tht you missed something that others did not.
3) Specify payment terms up front.
4) Last and most important, make sure that you include terms and conditions that define your liability, payment schedule, hours of work, and that the proposal is a unit anc cannot be broken into individual parts.
Hope this helps
AC