Ranch land that is for sale always seem to high at the time, but you better get it bought while you can, especially if it borders you or is surrounded by you. I can almost guarantee you that five years (or less) from now you will be glad you bought it.
Remember, it is an INVESTMENT, not a cost. You don't need to pay for it in one year, if it will be difficult for you. And look at what it will add to your place; I'm sure it will make your place more valuable.
I too, don't think $300 an acre sounds too out of line, especially when it is surrounded by your land. Even here grassland has been bringing $300/acre or more.
What if you didn't have the OPTION to buy it; what if he just sold it to
someone else? How would you feel then? He could, you know, sell it to someone else. Some people just want to own some land in Montana. We have a couple of 10 acre pieces like that and the owners (In California) won't sell it to us. They get nothing out of it and one 10 acre parcel is next to the road so they have access. It has not been a problem, but we'd sure like to get those pieces bought so that someday there is NOT a problem. One family, in Washington was very obliging and they quit paying the taxes so we could get their land for back taxes.
And easement issues can cause all kinds of problems. I know a case in Wyoming that sounded ridiculous because the folks wanting the easement had access to it on a different side, but they wanted to go in and out through this old ranchers. place. They pushed it and pushed it in court and it went on for years. Just about killed the old rancher. I don't know who won in the end, but I know it was a terrible thing to go through.There were lots of articles in the paper about it, as it was a HOT topic.
My vote is to get it bought. I think it is the very best option. Things will work out.
Hope this helps!