Opensource is an ideology that information and resources should be available freely to everyone should not be in the hands of few to make commercial gain. This would also suggest that the developers of opensource softwares are not looking for commercial gain but are working towards their ideology.
As an example, take up the case of Open Office. Since the developers did not want the product to be commercial in anyway, as soon as Sun was bought by Oracle most developers left the project since the suspected that the project would no longer remain non-commercial and fully opensource. They instead developed another open source office suite by the name of LibreOffice.
As for the funds to assist them in their development projects they mostly rely on donations.
Hope this explains.
B.R.Bhalla & Associates