Zack Arias has some great insights on this question. Insights which have encouraged me to stick with my current job and consider photography highly satisfying supplemental income. Full disclosure - I have only just reached the point where jobs come in regularly. They're sporadic and high-stress, but they're flowing. Note also that I don't consider myself a pro, but I'm just starting to see encouraging signs from my probably-lifelong trek toward that goal.
1] I'm a mechanical engineer.
2] About two years of shooting and six months of low-intensity marketing. I still have a long way to go.
3] Action, portraits, and events. There's little to no money in concerts because they can basically always find someone to shoot them for free, but if you have (most important first) a good portfolio of relevant work, experience, and connections, you can find well-paying work. Blogs frequently need photographers, but in my experience, they're also the most likely to underpay or attempt to screw you over. All agreements in writing, regardless of the client.
4] In total, about $5,500. Two camera bodies (60D and 1D mkIII), five lenses (11-16mm f2.8, 20mm f2.8, 50mm f1.4, 100mm f2.0), three hot shoe flashes and light mods, battery packs, light stands, tripod, extra batteries, polarizers, duct tape.... The list gets lengthy rather fast. Add $1,800 for my computer (which is also used for resource-heavy engineering applications). Again, Zack Arias has a
Kelby Training tutorial on professional photography with a $5,000 budget. Haven't seen it, but it looks like it's right up your alley.
Good luck. It's brutal out here.