Context's prices aren't bad. Higher than most, but in line with the quality they offer. I would say that for tourists who aren't specifically into whatever they're viewing as a hobby could be just fine with a local who happens to be a tour guide. Knowing more than the tourist is generally plenty. But if you're really into the venue, art, food, whatever, using an agency like Context will likely be better as it will be in line with your expectations.

For instance, I could take an art tour with just about anyone. I'm not particularly into art or knowledgeable, so anyone that can give me the high level info while we look at things is fine by me. But I love food, and food tours, especially on my first time somewhere - so I want a great guide that knows more than I'll know by the end of the trip.

I will say that Italy is one place where I'd recommend a food tour in every place you visit. Particularly for Americans who are mostly exposed to Italian-American food in the US, food tours in Italy really get at the very local and regional styles.