I'm trying to get an idea what to offer my friend and neighbor for a permanent easement to allow an irrigation lake to back water onto his property.

Here are the details:  I recently purchased 150 acres that joins another 200 acres that I already owned.  The two combined properties form a horseshoe around the neighbor's 25 acres.  The neighbor's property is all pasture/timber and they run 5-6 horses on it.  I don't think they have any ponds on the property.  Prior to purchasing the 150 acres I surveyed for an irrigation lake that I want to build.  Somewhere in my original survey (probably due to a lot of brush in way) I must have made a mistake because it is now apparent that in order to get enough water I'm going to back the lake up onto the neighbor's property.  I'm not talking about flooding a chunk of their pasture.  It would just back up into a gully about 700'.  The ditch or gully is steep and deep so would contain the water to about 50' wide or less at the property line and within a 100' or so just raise the water level in the current ditch w/o actually flooding any additional property.  I'm estimating we're talking about a maximum of 3/4 of an acre.  Obviously, the level would drop when we start irrigating and due to COE restrictions would remain drained until mid Feb. each year.  Property values are in the $2200 - $2500/acre range here.  Doubt if they would be interested in selling me a couple of acres though. 

So - what should I offer my neighbor as compensation for an easement???  I can see a small advantage for them in having some water available for their horses - but only for a short time each year.  I don't see a lot of disadvantage for the neighbor but fully realize that it is their property and certainly respect that.  I have not talked with them about this at all yet.  Just wanting to get an idea first so I have a fair offer in mind when I visit them.