Audio codec analysis in MATLAB: crosstalk
Most lossy stereo encoders (MP3, AAC, etc) operate in a joint-stereo mode, i.e. there will be a lot of cross talk between the two channels specifically at higher frequencies and in general they will allocate more bits to the sum then do to difference in the channels which also introduces cross talk.
The amount of cross talk will depend heavily on the signal and the bit rate. There isn't an east metric for it.
Subtracting the decoded version from the original is also not a great idea since you can't easily distinguish the encoding/decoding artifacts from the cross talk.