In keeping with its low cost heritage, Walmart is introducing a somewhat compelling offer for prepaid wireless service. Their latest offer, through their Straight Talk Wireless unit, provides unlimited talk, text, and data services on either an Apple iPhone 4 or 5 for $45 per month, with a catch or two.

New customers have to buy an iPhone at full price, ranging from $449 (iPhone 4) to $649 (iPhone 5), but Walmart is also offering an interest free payment option of $25 per month for the phones. It doesn’t look as if customers can bring an existing iPhone to Straight Talk though. If they choose the payment plan, their monthly cost goes to $70/month (until the iPhone is paid off).

“We believe customers shouldn’t have to choose between saving money and having the latest technology,” said Seong Ohm, senior vice president of Entertainment for Walmart U.S. in a press release. “Now customers can have the coveted iPhone with unlimited talk, text and data without a contract for $70 a month thanks to our exclusive Straight Talk plan and industry first financing offer.”

The $45 per month fee for unlimited talk, text, and data is significantly less than customers can find from other carriers. Walmart claims the deal can save new customers up to $950 per year. For example, a similar unlimited plan on Sprint costs $110/month (which includes a subsidized iPhone for an additional $199). Neither AT&T nor Verizon offer an unlimited data plan to new subscribers.

Besides the ‘catch’ involving the cost of the phone itself, GigaOm’s Kevin Tofel reports that this particular plan will not include access to 4G data services, relying on 3G, at least for the time being.

Straight Talk is a joint venture between Walmart and Tracfone Wireless, and buys capacity from various carriers for their prepaid offers. The $45 plan is sure to entice not only budget conscious buyers but iPhone enthusiasts as well and may help accelerate the already widely established cord cutting trend.