Well, 1 acre = 43,560 sq ft. So 5 acres is 217,800 sq ft.
A perfect square would be about 467' each side. so a perimeter of 1868'. The more rectangular it is, the longer the run will be.
How far apart do you plan on spacing t-posts. And what are you trying to contain?
Assuming a T-post every 10' you would need 187 of them.
Tractorsupply.com has the lighter weight 6' t-posts for $3.79 ea. So about $4 with tax. So thats ~$750 for posts
Have no idea what you mean by poly rope? is it a rope-fence material? Just a single strand of rope around the fences? what?