Cashed my collection out for around $3,500 when I stopped playing paper in 2008. I had a Mox Emerald, a playset of Mana Drains, Playsets of all Revised Duals, Juzaam, Playset of FoWs, and pretty much everything you can think of outside of the 8 power I didn't own. In retrospect I really wish I had held on to them, since Legacy has become popular the dual lands and FoWs and stuff have doubled in price (I think people pay like $150 for a playset of duallands/FoWs now). If I were selling my collection now I don't think I would have had a problem pulling down over 5k for it. *sadface*
I've spent about $500 on modo since I quit playing in paper though.. M:tg like EverQuest are 2 of my ultra nerdy passions I'll probably never let go of. Both are still going strong 10 and 15 years after I started.
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Edit: Lesson: If you have Magic cards that aren't garbage, especially old ones DON'T FUCKING SELL THEM! Two years from now you'll double your money, 5 years from now you'll triple it. I don't think the game is going away anytime soon.