My neighbor is the director of the land trust that operates in our area. They are funded mostly by government grants and facilitate land conservation easements with landowners. Generally speaking, most people are willing to go through the process to create a conservation easement if it at least costs no money out of pocket to them to do so. I was told 100 acres is usually the break even point for our area. This might have more to do with the resources available to the area land trust, I'm not sure. I think the land trust generally absorbs the legal fees and advocates for the change in zoning at the local level. The landowner gets significant tax credits for each acre conserved, with a value assigned according to the type of terrain. If the tax credits exceed the amount the landowner could make use of, they can be sold off at about 80 cents on the dollar to anyone willing to buy them for a 1:1 reduction of their federal taxes.
Friends of mine operate a farm that is located on land protected by a conservation easement. There were restrictions placed regarding how much land could be disturbed, building locations and size, etc. These were all negotiated beforehand.
There are 70 acres behind my house that the owner uses for a smalltime lumber operation. He has trails throughout and harvests fallen or dead (mostly ash) trees, that are then cut on his one man sawmill. He wants to place a conservation easement on it, but there has been some friction regarding what that entails. Neither he nor the land trust director will tell me details, but the limited size of the property came up as part of it. That might not be the main factor though.
OP, I would start by talking with your local land trust. They can walk you through the process, who pays for what, and perhaps have suggestions of properties that have the biggest impact through a permanent conservation easement. I may have some of my information wrong since what I learned comes from casual conversations, but if you have specific questions I can ask, or I can send you information through a private message.
edited to add-- To get more involved and educated, when contacting the local land trust, ask about volunteer opportunities. My group has an email listing that they advertise fundraising events and tree plantings. Spending a day planting trees with one of the employees of the group will allow you talk for hours about how it all works.