Granted, each application may have different features that may require way more memory than Goggles Music Manager. After all, GMM only indexes all the track information and does nothing fancy like album art and such. I'm not sure if these numbers are at all scientific. Still they're fun to look at. The numbers were taking after starting the application and starting playback.
(application) (VmSize) / (VmRss) / (Num Songs)
GMM 85 MB / 23 MB / 10606
RhythmBox 116 MB / 29 MB / 9005
Quod Libet 107 MB / 51 MB / 10545
Amarok 137 MB / 54 MB / 10562
Banshee 161 MB / 77 MB / 10263