While an mp3 player can play sounds in a linear fashion the 'grunt' of the DCC sound chip is in the IC that allows sounds to be polyphonic, modified, combined and also respond to parameters like load on the motor etc.
There are a few factors which make the G scale sound decoders a little more expensive than OO gauge besides the large factor of economies of scale.
They have heftier components that can take the higher currents involved. They may have more capable function outputs and also larger 'stay alive' capacitors on board etc.
The prices of European makers 'straight' G scale sound decoders is a little high but sometimes it is possible to combine OO scale DCC sound boards with G scale 'power' decoders for a cheaper outlay.
There are also some cheaper OO scale power decoders which can handle up to 3amps. So for some locos they can be decoder-ed with a combination of power and sound decoders for less than a ton. eg Digitrax, Uhllenbrock etc. Some combinations need SUSI connectors, some don't.
Of course the fly in the ointment is what sounds you are after as this can determine what options of price reduction by combining you can achieve.
For power thirsty locos...
The 6 amp (10 stall) G scale Titan sound decoders from the likes of QSI etc are very capable beasts and sell for the equivalent of about £125 in the states whilst their 10amp (20 stall) jobbies go for about £135. This is far more reasonable than equivalent Massoth XXL combined with Emotion LS sound decoder or even ESU over in the US...BUT over here prices, although better for QSI over its competitors, are not so distant from each other.
While the Hornby strategy might well bring down the prices of OO scale and even the O scale sound decoders overall, it is quite a jump for manufacturers to carry that over to the much smaller market of G scale...but hope is always good...... Soundtraxx might get back in the G scale market (good and cheap decoders) but maybe that will not help the search for cheap european sounds.