Man Cave or Casida

[Porter Rappleye]

I've got a detached garage I recently converted to a second living room. It cost me appx $5k to dry wall, carpet and tile the room.  Prior to beginning construction I had a vision of the ultimate "MAN CAVE". Endless parties, intense ping pong tournaments, and 24/7 football.  Unfortunately my vision of the "Man Cave" has fizzled into nothing more that a cluttered storage room.  

Since finalizing construction  of said "Man Cave", I've recently learned about a lot of other people in my neighborhood converting their garages into Casidas, which typically rent for appx $400-$500/month.  In order for me to do this I would need to drop another appx $7k into plumbing, shower, kitchen, etc..  So my total investment at that point would be $12k which at $450/month (average)  = $5,400/year which is a $5,400/$12,000 = 45% return on my investment (not including expenses). Since I live in the main house and will charge the tenant for all utilities I'm not expecting too many expenses (maybe i should??).  

My question is, do I keep the "Man Cave" (storage space) or do I convert it into a rental unit??