I bought the 2009 25th Anniversary reissue. The main difference are the printed

on Prime's arms and the shortened smokestacks. I bought it for arond 80$ brand new, shipping included.
To make him more authentic, you can always put the

stickers on the printed

on the arms.
As for the smokestacks, I bought a 10$ modkit for him. These are DIE CAST METAL so they are better than the original! It was a bitch to mod because these arms are full of little gears and springs. That's far from today's simplistic (but oh so efficient) ball-joint!
But if you don't care about the box and you can handle a little play wear, You can always buy a loose original and complete Prime that fit your budjet. Optimus Prime IS NOT RARE. Because he was SO popular. So he's easy to find and relatively affordable.
However, if you are a hardcore collector and money is no object, try to get the REAL ORIGINAL:
M.I.B. Diaclone Optimus Prime with his little mens and everything.