I agree with Glenstory, in that the price will depend on a variety of different things. Is the cow that you are considering selling the flush in proven? Does she have offspring that have been top end? Is she from popular lines, or is she outcross genetics? Are you guaranteeing a minimum number of grade 1 embryos? Are you controlling the time frame that the buyer has to do the flush? Are you allowing the buyer to get all the embryos produced, or are you keeping any over a certain number produced? Are you demanding who does the flush? Are you supplying semen for the flush?
I have sold several flushes in the past few years, and I try to keep the rules and regulations as limited as possible. I offer a minimum guarantee of grade 1 embryos and if they buyer does not obtain this number, he has the option, of only paying for the percentage he received, performing another flush, or taking some other embryos to make up the difference. I have had buyers do all of these. The one thing I do request is that my ET vets do the flush, or at least, I want to have approval of the ET people doing it. The reason I do this is simply because I have seen donor cows nearly ruined by certain ET vets. Several years ago, I had a local vet flush 4 cows for me, and we were never able to get any of them to rebreed or even flush again. After a few thousand dollars trying to get them back on track, they were eventually sent to market. Maybe it was a one time deal, but I cost me a big bunch of money.
I would suggest that a flush should be valued in the ball park of one of this cows best calves value. Offer a minimum guarantee, make sure all the terms are understood by both parties. Usually terms are 1/2 at time of purchase, 1/2 at time of flush.