I made a (new) income simulator for the game, it might help some people...
http://www.monsieur-t.fr/pingy/
Many thanks to the devs for the help with some formulas, and of course, any comments and suggestions are greatly appreciated !
I quickly made a spreadsheet showing the average money/exp income from various activities in the game, if you guys want to take a look it's here : https://docs.google.com/spreadsheet/ccc?key=0AjszXWbJOiLqdERjVDEyMURZS0E5aWJ4al9rNDZXSkE
Unfortunately, since the market prices are level-dependent, it might not be relevant for everybody.
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+1 for this, its a great tool, and really easy to use :D