Couple things would help frame it properly--when you say .89 for over 1K, and 2.50 to 3.50 on smaller orders, how many colors are you talking about?
The thing to keep in mind is, if you are having issues because of your press, then a new one will fix it--the trick is *knowing* that the press is the problem. I've heard of more than a few people who had registration problems, that then traced it to their films, or low/uneven screen tension. I'd also point out, if you can't hold tight registration on your manual, it will stink to set up every sub-48 shirt job on the auto because it's the only press you have that holds registration--I think of a good manual as a necessity, personally.
The other thing that's easy to forget about is all the random things you'll want to upgrade to keep up with the new auto--Do you have a good film output system, a compressor and dryer that will supply the press, that MSP exposure unit, good screens, a properly sized washout booth, proper electrical service, etc?
Just to look at a simple count--if you're looking at .89 for the big orders, if you figure 15,000 shirts for the lease payment you're talking about, you'll be paying twice as much for the equipment as you will have left over for your materials, labor, and (hopefully) some amount of profit. Not the greatest numbers. If you have a lot of prospects, this may not be the right argument--but again, there's that old adage about counting your chickens...
If you can outsource the 1K plus orders you can't handle on a nice new manual, you'll likely be *clearing* a lot more money, and I guarantee you more manual experience will absolutely pay off when you move to an auto.
Don't by any means take this as a 'don't buy an auto' post, just make sure you have the bases loaded before you try to knock it out of the park.