Not to be outdone by retail stores like Best Buy and RadioShack, Sprint itself is said to start offering a major discount on its iPhone 4S this weekend.

TechnoBuffalo is reporting that it has learned that the carrier will soon launch a trade-in promotion that will knock the handset below $100…

“If you’re an iPhone customer considering jumping ship to Sprint, the Now Network’s upcoming promo just might be enough to influence your decision. A trusted source has share with us that starting on May 18th Sprint will begin offering a minimum of $100 on an iPhone trade-ins towards an iPhone 4S.”

This means that you could trade-in an old, rundown iPhone 3G or 3GS and get Apple’s latest smartphone for $99 (or lower). That’s the lowest price we’ve seen on the device to date.

As far as Sprint goes, it’s taken quite a bit of heat from iPhone customers over its painfully slow data speeds. So you may want to investigate that in your area before you sign up.

But it’s also worth noting that it’s currently the only major US carrier still offering an unlimited data plan, and plans to continue to do so,¬†even when that rumored LTE iPhone hits this fall.

The $100 trade-in promotion is set to run through June 30th, and as with all wireless deals, will require you to sign a two-year contract.

What do you think? Deal, or no deal?