It's one of those semi-hidden server values things. I think it's generally around 1 point per square (at least for hyper and sublight). Might be slightly different in atmo/cities. But XP is a peripheral bonus of moving around, and not meant to be the purpose for moving around. Trying to work out how much you get from what, and thereby tailor your travel to maximise XP, is bordering on a violation of the Golden Rule.
Of course you might argue the same about people who just fly around constantly for the XP. But my point is that you shouldn't worry too much about how much XP you get from what, and just enjoy it. =D
"May the Grace of Ara go with you, and His Vengeance be wrought upon your enemies."