Sprint’s Boost Mobile prepaid division will start offering its $50 Monthly Unlimited plan on Sprint’s CDMA network starting Jan. 13, and it will add its first smartphone to its line-up as well.

Boost did a trial of an unlimited CDMA offer in select markets before its unlimited iDEN offer was introduced last year, but that offer still had taxes and fees and wasn’t as simple as the $50 offer. Boost says $50 is $50 – and no other taxes or fees are added to the bill.

Historically, Boost offered Motorola handsets compatible with iDEN, and Motorola was its exclusive handset supplier. With today’s announcement, the company is broadening its line-up to include not only a BlackBerry but also two Kyocera Sanyo models, the Incognito and the Mirro.

The BlackBerry Curve 8330 from Research In Motion (RIM) will be available on a new $60 monthly unlimited plan. The Curve sells for $249.99. The Sanyo Mirro is priced at $99.99, and the Incognito is $149.99.

A year ago, Boost introduced the $50 per month unlimited talk, text, Web and walkie-talkie plan on the iDEN network. The company says that offer drew more than 2 million new customers in 2009. In the third quarter, Boost says it attracted more prepaid net customer additions than MetroPCS, Leap, T-Mobile prepaid, AT&T prepaid and Verizon prepaid combined.