Ethical and sustainable volunteer opportunities
Although this organization also has fees, they are not as high as GoEco and Operation Groundswell. I saw a few listings that were around 500 UK pounds (as opposed to several thousand USD). Moreover, they seem very concerned with ethics and sustainability. For example, they employ a "careful and independent selection process, ensuring that we only work with sustainable, long term projects that we know personally and where there is a lasting, positive impact on the local communities". Check it out and see for yourself!