Depending on which mill you end up purchasing, will determine how long before it pays for itself. I wouldn't make that the only factor you consider when looking at mills though. Personally, I wish I had gotten the one I have now the first time. I went with a cheaper one (Barley Crusher) that did a decent enough job. But then I had a chance to sell it to another HBT member, and I ordered up the Monster Mill... Wish I had gone that route from the start. I would have spent less, total, if I had.
I'm sure there are going to be plenty of people who will say how the Corona mill is perfectly fine for milling your grain. Just as you'll find people loving the mill they have at this time. If you have a chance, check out the different mills in action before you make a purchase. See if there are members close enough to where you are, that you can go over on a brew-day to see how the mill they use works. Then you'll be able to make a more informed decision.
BTW, milling your grain on brew day, as you start brewing, is priceless. The longest my grain sits, crushed, is about an hour. Then it's into the mash tun. :rockin: