Oh I thought the point was that donating money has more impact than donating time? Back pedaling are we?
There are many many flaws in your logic. The average difference in the quality of work put forth by someone who is doing a task for free to help others versus someone who is being paid minimum wage is significant. Many people have more time than money to give. (Excuse me while I go pick $100 off my money tree!) Not all "charitable" organizations use funds wisely/ethically, hence the need for sites like
http://www.charitynavigator.org/. One can volunteer in the medical/dental field at non-profit free clinics. You should not be comparing the value of man hours in the US to a salary in another country, as most volunteers in the US will be volunteering in the US. So my time is worth at least 7.25/hr to an organization. Not all worthy NGOs are international, you know.
You can't really say who makes a "larger difference" unless you are speaking strictly in terms of dollars, in which case of course "big donors make much larger difference in one to one ratio of donor to volunteer" (although depends on hours volunteered doesn't it?). However, a volunteer who cares about the people they work with is more likely to listen, show respect, provide help, and go above and beyond than someone who some rich guy paid to do a task for minimum wage. A volunteer can change the life of someone they help in many ways that writing a check simply cannot.
Volunteering in certain settings can increase one's level of empathy. We need more empathy in this world.
Anyway, I could go on, but I think I've volunteered enough of my time here.