How much would a 100KW solar and battery system cost?

It depends on place where do you want to build it and how much money yu want to spend and how reliable you want it to be. Strange isn’t it?

1 m^2 panel will give you about 200W of power when in full Sun. So you will need 500 m^2 of panels to get to 100kW power output.

But on average you can only get 4hours of full Sun a day. So you have to multiply that 500m^2 by 6 to cover output for whole day. that gives you 3000m^2 only to generate base electricity. Then you have to account for battery efficiency to charge and discharge. That’s about 75% so you have to add that more panels to cover that loss. It takes you to 4000 m^2 of panels.

And I did not continue to count possibility that there will be overcast for several days, so yo have to have more batteries to cover that period. And to charge those batteries you have to get energy somewhere (that means another panels).

There is plenty of room where you can get more effective solution but it will cost you money to buy it and more money to support it (for example lead-acid batteries have more live span than Li-On ones but are less effective and take more space).