A good rule of thumb, is that you are a sucess if your first test mailing breaks even. So don't expect to actually make money the first time around. You are really prospecting for donors. The money comes from being able to go back to these people in 6 to 12 months and asking them to give again. There are other things too.
Handwritten envelopes get a better response that labeled envelopes.
First class mail does better than bulk rate. An actual stamp does better than a postage meter. I am impressed with your candor regarding the proposal you are planning to submit to the educational group for a direct mail program. Depending on the responses to those questions, you should be far better able to determine if a direct mail campaign for this prospective client is really the thing to do at this time, if at all. Your good judgment is required to understand the real value and position of direct mail fund-raising in the development plans of this particular organization.