The planet income shown on the planet page, is there per month income for all the facilities or a year or something else?

Eg. http://rules.swcombine.com/?Galaxy_Map&planetID=27

Is that 1.8bil per month?

Income is paid monthly.

http://rules.swcombine.com/?Facility_Income

“Income is distributed at 05:00 GMT on the 1st of every month. ”

And yes, your assumption about the 1.8 bil. per month is correct.

Thanks for the help.

Planetary income has absolutely nothing to do with facility incomes. It's just a figure from which tax income is derived. Tax income (the bottom figure) is issued at the same time as facility income.

So the combined facilities on Coruscant don't make over 1.8bil per month?

Not unless there's about 150 high income facilities per city..