First, it aint easy. If it was, everyone would do it!
We opened our shop almost two years ago now. It was the best, and worst decision I ever made.....

depending on what day it is of course. What it will cost you will depend on how much you already have in the way of tools.
Fortunately, between myself and my business partner, we already had most of the tools we needed. You will need things like mig welders, air compressors, hand tools, air tools, specialty tools for engine building, etc.
The depth of tools you will need will depend on what your shop will be able to perform. Fabrication? engine building? or just head and cam installs, header installs, etc.
will you have a chasis dyno? That was our biggest expense when we opened.
Lifts? how many?
We opened a 3k square foot shop with an in ground chasis dyno and three lifts. We were able to do it for right around 75k. But again, we already had a lot of tools. We lease our space but I will be looking at buying it in the next year or so.
dont forget expenses like signage for your building, advertising (that can get quite expensive depending on how many sites you sponsor) etc. you will need access to parts and buying into major companies to carry their products aint cheap either.
I can tell you, this has been the most challenging thing I have ever done, but WHO CARES. Cars are my passion. I love them. They intrigue me. I love what I do and always wanted my own business. If its what you want to do, then find a way to do it. Make it happen.
I would suggest (and I know some business owners will dissagree with me on this one) going into business with someone you trust as a partner. Its very difficult to do it all on your own. In my case, my business partner and I are 50/50 owners. We each brought things to the table that were absolutely necessary for the business to succeed. Its not easy at times. There are days we want to strangle one another. Days when we thing about sabotaging a lift so it falls on the other guy!!!

But each one of us knows it would have never been possible without the other guy.
Sorry for the long, rambling post. Good luck.