How old are you? Well, I assumed you're not old enough to have a job/earnings to pay for you own iPhone...
Let's see... where exactly do you live? iPhone prices are vary, depends on which country you're living in. If you live in the states (U.S.), then the lowest subsidized price of an AT&T's iPhone 3G is around $199.
First step, you print out the picture & specifications of the cheapest offered price on iPhone 3G you can find. Doesn't have to be from on-line stores, local stores usually give better price.
Second step, you search and print out another smartphone with higher price tag. Let's say Palm Treo Pro that's offered somewhere at $475. For better percentage of success, I suggest you find another one that priced between the iPhone 3G and the highest price tag. $250-300 sounds confincing.
Next, you show them to your parents. And try your best to keep on mentioning the benefits of iPhone 3G compared to the others. Such as: bigger screen, longest battery life, multi-function device (can do calls, text messaging/SMS, play music & videos, etc); and most of all it has the cheapest price tag!
If that still doesn't work, then use the old trick once again to them: you let them use it first for a couple of months before passing it on to you. ;-)
Wait, better yet: drag them to the nearest Apple Store and let them play few minutes with an iPhone. Who knows? They'll like it, and even buy it for themselves too... -LOL-
If you need more reasons, you can read the URL link I posted below my answer.