making targets, a then shooting the heck out of 'em, is one of the most rewarding parts of the shooters experiance. find a scrape yard, or material recycling yard, and you can find steel plates in all sizes for really cheap. last i herd, steel was 70.00$ a ton. that was a long time ago, maybe even 10 years or so. im sure thats changed. but for what you are looking for, should be cheaper than 100.00 + shipping. you dont need anything special. (i.e armored or hardened) i would look at maybe 3/8 ths inch to 1/2 inch thick. .223 can push a bullet through 1/4 inch steel at 100 yards. .308's can push a bullet through 1/4 inch at 200 yards. 300 RUM can push a bullet through 1/2 inch at 350 yards. (max range we could test) could maybe go further.
if you make a target, and are able to make the center "hit" part of the target a movable, or swinging piece, your target might last longer. make sence?
i hope this helps a little. just think, if you dont succede, just lean it up against a tree, shot it, and when done, try again, or buy an already made target. i thin khtat you may find it more fun to make your own though. brian