Cheap guided tours by locals (St. Petersburg)

I found new nice startup for those who come to St. Petersburg and want to take a guided tour in English but not too touristic one.

By the way, I myself took such tours in Berlin last weekend: from New Europe and Alternative Berlin.

Here you can have the same in St. Petersburg: http://www.sptnik.com/

For example, roofing (walking on the roofs) experience for $ 21 or futuristic buildings for $12.
I'd say it's a good catch.