You are easily looking at 6 digits even if it is a small datacenter. For one, a datacenter isn't just any good ol' building.
You need to install for example high end security, fireproof facilities, generators, UPS, raised flooring, a NOC, invest in routers, monitoring software, and such. In general, you need to ensure
1. Physical Security
2. Redundant Power supplies
3. Disaster Proof
4. Fire Proof
If you are in a earthquake zone, you would also want to make sure that it is certified for up to a level of quakes (though the best is that it's really not in one)
None of the above is cheap. And this are just setup cost. You still have monthly maintainance and more. And this is for a small setup. If you look at a bigger setup, it can easily bloom into the 7 digit area.