If your neighbor owns the lot beside you and will add the 15' to his property is has a lot more value than if he just wants it to have rights to launch a boat or have a family picnic. You should call a local real estate agent and ask for a free CMA. They can tell you how much more your neighbor's house would be worth with the additional piece of land or how much less yours would be worth without it.

Ultimately, the fair value will be whatever the two of you can agree on. I would tend to agree with you that the value of the land should not come at a cheap price. If your neighbor is truly motivated to buy the land, then he will need to realize that the opportunity for him to increase the size of his lot is worth the additional cost.