Base maintenance costs are not usually a massive part of your total expenditure, however when you first get to the point of having all eight bases you can build, you may find they can rise to be at least a slightly noticeable drain on your finances for a while.
Due to two bugs it is not easy to tell more than generally what your base maintenances costs are unless you add all the numbers up yourself by hand. At the end of each month the value shown on the Finance graph is accurate, but you will only probably get an idea of the total costs from there as that also includes wages and craft rental, so you would have to remove them. The values shown are the graph are stored in LIGLOB.DAT for a more accurate starting point than you are likely to read off the graph itself.
The first bug is that the way maintenance costs are calculated to show you on the Monthly Costs page are incorrect, so should not be used at all. The other known bug that affects this is more serious - any time you knock down a facility or cancel one in progress, the dirt squares will cost you 80k each (and therefore 320k for each Hangar removed). This cost will be immediately in effect if you tore down a completed facility, or will come in at the point when a cancelled facility would have come on line.