Don't forget you still need a backup drive no matter what you choose for the new iMacs internal configuration. So if your numbers ar correct the 1 TB SSD would cost you $450 for the SSD option and $178 for the USB 3 backup drive.
However, you numbers seem a bit high for me. First off, I really like OWC products, I'm using one of their USB 3/ FW 800 external enclosures for one of my backups. However, you should consider looking into just buying an empty USB 3 enclosure, and purchasing a drive for it separately. USB 3 enclosures can be had dirt cheap, and 1 TB drives are also dirt cheap. This is one way you can save some money by building it yourself. It is really easy to do.
Tom