Let’s take a brief look at what I would call the bottom-line total cost of ownership (TCO) of the original 8GB iPhone versus the forthcoming 8GB iPhone 3G. That is, the TCO of each iPhone with equal data (unlimited), voice (450 min), and SMS text messaging (200 txts).

(Above I’ve charted the initial cost of the iPhone + setup fees + monthly service over 2 years, including taxes and fees. I’ve added a $5/mo text plan and calculated the totals using California sales tax (8.25%) – so the bottom line in different states may be slightly more or less.)

More after the jump, plus another chart of TCO without a text plan…

The result seen here is that even given the lower initial cost, the iPhone 3G costs more to own than the original iPhone at 14 months and after 2 years is a little under $200 more expensive. This is due to the monthly 3G service plan being about $15 more expensive for unlimited 3G data + 200 texts.

If you are one of the few smartphone users that really, truly never use SMS text messaging, it takes until about month 20 for the cost of a iPhone 3G to surpass the original iPhone. Here the price is much more closely matched, though as mentioned earlier the original iPhone service includes a 200 SMS plan and the iPhone 3G plan has none.

Personally, GPS and 3G make it compelling enough for me to upgrade. Plus I suspect the original iPhones will still be worth a pretty penny (easily $200+) on eBay even after the iPhone 3G is available.