FOSS in no way means "free of cost". We use a piece of FOSS software where I work, and it costs 4 figures per seat.
FOSS means free, depending on what you want from the software - if you want more than the FOSS offering does, you may have to pay for improvements, or you may not.
I can't help you with your disbelief, but it is true that nothing in the FOSS philosophy prevents companies from charging for their work.