I'm new to chickens, but not to fencing.
6-1/2' heavy T-posts are about $6 here, and while a landscaping timber may only be $3, you're still going to want to cement it in the ground. I mean, I would, if only because pouring a hole full of dry mix is easier than backfilling and tamping with the half-dollar sized flat tamping end of a spudbar.. What a pain that is.....
You may be able to get by with $1.50 worth of concrete and a bit of tamping, but then you're still up to $4.50/ea -- and you had to dig a bunch of holes, deal with a lot of heavy, dusty concrete, backfill, and tamp! I dunno about you, but if someone offered me $1.50 a pop to do that much work, I'd kindly tell them to take a long walk off a short pier. It's just not worth it.
If it were me -- and it has been, several times -- I'd spend a few extra bucks on real wooden fence posts for your corners, set in cement, and braced (either H-braces or 'knee' braces). Then, stretch your fence from corner to corner. After the fence is up and stretched, drive t-posts about every 8'-10' along the line and 'clip' (wire) them to the fence. I might set a 4" wooden post in cement every 50' or something like that, just for a little added stability.
Stretching the fence is very, very important.. If you use t-posts without stretching the fence properly...yeah...the t-posts will get wobbly because they're trying to hold up the heavy fence. I've seen that a lot. Now, I know it seems like holding up a fence is what t-posts are supposed to be doing anyway, but not really... If your fence is stretched properly, it will pretty much hold itself up and the t-posts are there to help keep it from twisting in or out, and to keep things from being able to walk under it. For that purpose, they work amazingly well -- and they're FAST!
Oh...one other thing that will help you in the long run is to fence when it's HOT outside. It's a pain, but I've seen fence stretched in the early spring, when the fence wire's cold and contracted, and it's loose and wobbly by midsummer. If you stretch it in the heat, however, it only gets tighter from there.
Good luck!