Depends what materials you are going to use to fence the property. You mention you have trees, but what kind not all trees/wood can be used to fence with. Are you going to use barbed wire too?

It can also depend on what you want to fence out.
If you just want to better define the boundaries…
Or if you want a fence high enough to keep out deer…
Or narrow enough to keep out dogs.

If you’ve the equipment and the trees, I’d say do with your own wood.

Most people who fence 40 acres use a combination of wood posts set at 100 foot intervals with metal tee posts in between, all held together with 3 strands of barbed wire. To figure up the costs, you will have to calculate how many tee posts you need, the cost of a post digger, the barbed wire per foot, and the cost of a wire stretcher. A farm store can provide you with those costs, since they will vary from area to area.

I’m including a link to a pdf publication from the University of Iowa extension office in which someone actually did a cost comparison of the different types of fencing.

If you have enough trees, why not make the “rails” too. Then it’s just your labor.