Hey there,
I know exactly what you went through. I had the same experience.
I volunteered in Ghana for 5 weeks because that's all the time off from work that I could take. I went through a grassroots organization that provides low cost volunteer opportunities in a few countries. They are not a registered non profit at this time but are very well organized.
The cost for up to 3 weeks is $250, one month $350 and every month thereafter an additional $100 per month. It's great because you are essentially only paying for your room and board. Of course any additional contributions you can bring would be great but not necessary. You pay a $30 fee to hold your spot only because many people would make the commitment and then back out. AFter that, you pay nothing until you actually arrive.
http://www.kidsworldwide.org
I volunteered at Golden Era International School. Mr. Aboagye is the principal and you also stay in a second home on their land. Their family was AMAZING and very much like my own. He started the school and operates it. The children are wonderful and the family is flexible with your needs if you'd like to travel from time to time.
I have TONS of information on these guys and plenty of photos to share if you're interested. My email is actionmakeschange@gmail.com. I'm working on a project hopefully to install self composting, environmentally friendly toilet facilities. If you do go through kidsworldwide...and are interested, please try to contact me for more information on these. If the school you go to is in need of toilet facilities or upgrades, each toilet installed costs approximately $250-$300 US dollars and would make a FANTASTIC contribution. Kidworldwide isn't agriculturally based but you could be helping the area if you decide on a self composting toilet as a donation as the remains provide a nutrient rich fertile soil that can be recycled to fertilize crops. Doesn't sound very great and healthy but it is completely safe!
Let me know if you have any questions and best of luck!
Cassie