I have several of the DSX decoders...but haven't installed any yet. I bought them for two reasons. They were inexpensive and I'm planning on putting them into bashed models for which I don't need the exact simulation of a particular engine's sound. I'm also NOT into loud engine sounds. The din created by loud exhaust and chuffing just bugs me...just a personal thing. On the other hand, complete silence is also not desired.
I bought my DSXs from Tony's Train Exchange when he was running a sale on them last year. I think I paid around $30 to $40 for each one back then.
If I need more volume, the amplifier is $12....and an audio isolation transformer can be purchased for between $1 and $10. You need to ensure it's an AUDIO isolation transformer or you may loose the lower frequencies from the DSX. I hadn't considered the need for the isolation transformer, but as Bill pointed out the power for the amplifier needs to be connected directly to the battery...and NOT the airwire as it would cause a short between the battery and the DCC output on the Airwire...and will probably damage your AirWire receiver.
So...from a cost perspective ALONE...$40 to $65 for the DSX, $12 for the amp, and $10 for the isolation transformer brings the cost to $62 to $87 per engine....compared to around $155 for the P5. If you DON'T need the extra volume, then the cost per engine is just that of the DSX...$40 to $65 per engine. Then again, you're assuming the liability.
Now...the new news....the QSI Solutions product...and it's got me rethinking what I'm doing. I went though all the above to get the cost of all loco electronics down in the $180 to $200 per engine cost....$140 for the Airwire RX...and $40 to $60 more for the sound. NOW....QSI Solutions has released their new Quantum Magnum and Gwire RX which are compatible with Airwire TXs. I understand you can buy both the decoder and receiver for around $200 per engine...which makes this integrated unit price competitive with my home grown low cost solution. Further, I'm hearing from Greg Elmassian on chat that he REALLY likes the sound that the Magnum makes in his locos.