Lattitude Global Volunteering cost around £1200-1700 for 6months (not inc. flights), which I don't think that sounds like a massive rip off. You get their expertise, security of you placement and most importantly the safety factor. 24 hour helplines, someone in the country who's there for you and peace of mind for your parents who then won't ring you every day.... And if you do get terribly injured you know they're going to take responsibility for you, get you into good healthcare etc.
I think it's important to differentiate between gap year COMPANIES and gap year CHARITIES.
I do think many are massive rip offs....
As a rule, I think any gap year organisation who AREN'T selective of their candidates whether by application/interview/selection are probably more interested in your money than you. If a company is willing to send anyone across the world to represent the UK and take on a responsible role without checking they are actually suitable for it and just want to know you can pay for it... I wouldn't want to go with them.
And actually passed?