Nothing in WI, but I recently negotiated one in my area. In my case, the easement was 50' along the border of a parcel about 1/4 mile long. In the end, it comes down to how much the seller perceives he is giving up and how much it is worth to the buyer. Keep in mind, you will need to update multiple plats/deeds with the county, both for the buyer and seller land. Those transaction costs need to be covered by someone. Typically, a survey is needed as well. That generally means, even if the owner considers his land value diminishes minimally, the purchase price needs to be high enough to deal with the hassle.
My case was unusual in that the easement was granted to a property I did not yet own. I made an offer contingent on the seller buy an easement with my specification from the neighbor. The three of us met over lunch as gentlemen and discussed the options. The easement seller then talked to a lawyer and real estate agent to get pricing advice. In the seller of the property I was buying agreed to pay about $5K for the easement and cover all of the transaction costs for the easement seller. I agreed to cover 1/2 of the easement transaction costs at closing of the sale of the property I was buying.
The property I was buying already had an easement to it, but the cost of putting in a road along that route would have been much higher than my cost associated with getting the easement. It all worked out for me.
In the end, it is simply a matter of a buyer and seller coming to terms. I'd have a chat with the owner and see if he is amenable and go from there.
Our cases are similar in that the seller has incentive to grant the easement if it helps them sell the property. It will probably boil down to whether the seller feels he has a better chance of getting his price for the 100+ acres intact by keeping it together or whether he feels he will have a hard time selling it and he would rather sell 40 now and thinks he will be able to sell the rest later and get his price in total.
Thanks,
Jack