The original M&P15 was actually offered at a premium price, although I don't remember exactly what it was, the M&P15 Sport was the cheap one, which originally came in at $709 MSRP, but could be found in shops for around $450-$500. If you saw one for $389.99, then it was probably the M&P15-22.
Smith & Wesson has offered the M&P15 in several configurations:
- M&P15 — The base model, offered in a configuration similar to the popular Colt LE6920. (aka the Civilian M4 Carbine)
- M&P15T — The "Tactical" model with a freefloating accessory rail/handguard.
- M&P15 Performance Center — The "Precision Rifle" model with a 20" Barrel, rifle length gas system, and factory tuned trigger.
- M&P15R — A standard M&P15 chambered in 5.45x39.
- M&P15 Sport — The entry level "Sporting" model without a Forward Assist, Dust Cover, and a Nitrided 1/8 Twist Heavy Barrel in place of the Chrome-lined 1/9 Twist M4 profile barrel of the standard M&P.
- M&P15A — A variation of the M&P15 with a Troy Folding Battle Sight in place of the detachable carry handle sight.
- M&P15X — A variation of the M&P15 with a Troy drop-in Quad-Rai
Handguard and a Troy Folding Battle Sight.
- M&P15OR — The "Optics Ready" model with absolutely no sights included and a MIL-STD-1913 Railed Gas Block.
- M&P15 Sport II — An upgraded version of the Sport with a Forward Assist and Dust Cover.
- M&P15 Sport II MOE — A premium version of the Sport II fitted with Magpul MOE Furniture and a Mid-length Gas System.
- M&P15T2 — An updated version of the M&P15T with an improved pistol grip modeled after those on M&P 2.0 Pistols and a proprietary enhanced variation of the A2 Birdcage muzzle device.
- M&P15 Sport III — The latest version of the Sport line with a freefloating M-Lok handguard and a Mid-length Gas System.
Please note that there were also a multitude of generational changes between models such as early models of the Sport II having chrome-lined barrels with 5R Rifling while later models did away with the chrome-lining and 5R barrels as a cost-saving measure. Or later models of the M&P15T/M&P15X having Magpul M-Lok Rails and MBUS folding sights over the original Troy MIL-STD-1913 Rails and Folding Battle Sights.