I decided on a 60ft. You can figure out how many posts you need by taking the size of your wanted round pen x 3 (3.14) and divide it by how far you want your posts to be apart. I went with 10ft since I was using 2x4x10's.
I bought 21 decent landscape timbers as my posts ( I went with a couple extra just in case). They are 3 1/2x 4 1/2x 8 & cost $4. I then bought 25 2x4x10's at $3.50 (the extra were for the gate, and I had a few around the farm. I got some 1x4x10's for the gate as well (I had 1 on the farm). Then bought 1/4 inch nylon fencing. Some of those you nails, Cattle gate hinges, some chain & a clip, a small bag of self mix concrete, & some of that ground spray paint (so I could put a stake in the center where I wanted it, tied 30 ft of twine on it and made a circle with the spray paint.
The gate is 7ft long and 5ft high.
The concrete is for the 2 posts where the gate is at. I dug the holes approx 2 ft deep. I've included 2 pics so you can see how I constructed it. I also borrowed a friends skid steer for the ground, and used my tiller to till up the land. I eventually plan on putting sand in and a gravel driveway leading up to it, but this is what it looks like now.
I did all of this for under $400. :)