All in all, it depends on WHERE you're doing this. In TN, there have been a few pop up recently.
On average, a steel building thats around 25 x 15 is going to cost 12-15 grand.
You can do the foundation yourself for about 3 grand, IE: clear off a good sized lot, pour your concrete, etc...
Then you have to run power, water, some kind of heat (because its cold as balls right now). The cost here depends on your city and how they do things. Not sure how the plumbing would be worked into all that, but I'm pretty sure you don't want to have a portapotty on site all the time.
A 2 post lift (great for smaller sized garages) will run you alot less than the 4 post, and they make them to handle alot more than you'd think. The dude I used to take my Tbird to had one rated for 15k pounds. Thats enough to pick up anything that'll fit through the door lol. They run about 5g's.
Then I'd say spend a grand or two on a compressor system that can handle the work load of multiple machines being used.
An average tire machine - 2-3 grand new.
Alignment machine - 5-8 grand new.
You've already got some tools, but a full tool set would help you out a bunch. IE: pneumatic tool set - 400$ for a "full" set new. A "full" set of hand tools could run you upwards of 5 grand as well.
Engine code reader ~30 bucks for a basic one, but they have some fancy ones that'll run you a couple bills.
Storage for all this stuff is a pretty penny as well... I found this site and started drooling immediately!
http://www.garageappeal.com/ So shiny... so very shiny...
This list could go on for days... but thats pretty much the bare minimum stuff. You don't HAVE to have the alignment machine, but if people are going to get tires, or have any suspension work done... its a must imo.
Oh and some kind of welding equipment would be a plus.
So from what I've googled, and from what I've seen done... you could get a pretty decent shop set up for around 45 grand.
But thats just getting the building and tools. You've also gotta get some kind of a customer base, partnerships with dealers of all the different consumables you'll be using, some kind of advertising to help extend your customer base, etc....
Quite the undertaking.