Volunteering in Africa?

Hi, sorry in advance for the long question.

I'm looking for an opportunity to volunteer in an underprivileged country, preferably one in Africa. I would like to go with a Christian group, but can't find one that caters to my budget. I'm a college student who has very little spare cash, so while I can pay for a passport and vaccines, I can't pay for air fare.

I've volunteered for a local soup kitchen here in the USA for a couple of years and I love it, but I want very much to get in on helping any truly needy, starving, or education-deprived people, especially children.

I've looked at the websites of dozens of organizations, but the ones that don't charge an arm and a leg to get you there seem like they are catering to more prosperous communities that have consistent food and hygiene.

I want to go somewhere that is going to be very challenging for me. I'll admit that I'm going for selfish reasons, as I'm hoping to find inner peace for myself through denying myself of the luxuries I take for granted, and to exercise looking outside my own needs and wants to help others.

If you have any experience volunteering in impoverished countries please share it with me. Or if you think this is all just a fantasy and I should just go back to my soup kitchen, please say so.