You only save about $500 or so going from the SD5 to the SD3 module... well under 25% of the cost, so I doubt an Audya module is going to be the bargain anyone wants. If you think that that $500 or so is the cost of a keyboard and speakers, you probably aren't going to save much more, because that's all (hopefully!) that gets dropped on an Audya module.

Unless space is at a VERY high premium, I can't think why anyone that actually CAN play a keyboard would bother with the hassle of a module...