Military bases do not produce anything, however they greatly reduce per-turn military expenditures and private market purchase prices. Private market sale prices are also lowered as well. Military costs may be lowered further by acquiring military technology.

How do they actually work?
Does anyone have the exact figures/values?

The wiki took out like all the information it used to have in it, so you have to go look at the wiki history to get the facts.

Finally dug this up:
Price * (1-0.25*Bases/Land)
Meaning I guess if your land is 100% military base, you get 25% off?

Thanks for the info blid. ")

What about the -expenditures?
Military bases do not produce anything, however they greatly reduce per-turn military expenditures

and what about TPT?

The formulas are on the wiki h2orich...

This is a good page:
http://wiki.earthempires.com/index.php/Formulas

Constant = If MilBases/Land < .295 Constant = 1 - 1. 3 *(MilBases/Land) Else (that is, if the number of MilBases built is more than 29.5% of your land) Constant = 0.61

Tech Per Turn TechPerTurn=Round(0.17*Labs*(1+Labs/Land)+3,0)

Usually you will either have a Theo or Demo or Commy having a large amount of Military bases.

Never a commie.

the 10% market fee is killer right? aren't military bases usually built for a reseller so the ideal govt is demo? and isnt it something usually switched to near end of set? like dozing all ur labs or something, putting all ur stuff on the market at high price, then switching to demo MBR?

Yep. Theo MBR is typically better than Demo MBR (but not always), because Theo has a -25% additional government bonus for PM prices, so jumping as a TMBR from private is usually a higher jump and a higher resell margin of profit.

It takes shorter to jump the same amount of stockpile as a Demo (because military ~22% is cheaper in PM for a Theo, so it takes ~22% longer to jump), hence DMBR _can_ be better since a shorter jump means you can stock an extra day or two to compensate for it, particularly if you take full advantage of Demo's reselling capabilities while jumping from PM (still with a big pile of cash on hand).