I mentor alot of startups. This is exactly how I work. Also the peer to peer coaching i did with Joel from Buffer uses similar principles. we only asked each other prodding questions but never gave an actual answer. It was all about leading them to a discovery that could work for them.
It's a fixed fee per session. Some start-ups like to have a session every week/2 weeks, others prefer to do it on an ad-hoc basis. The pricing scales based on the size of the start-up.
If you'd like a free session as a trial you're most welcome. There's no obligation or commitment necessary.
I devised the system and iterated it with Joel. It was originally inspired by the "toyota way" but i've just read up on the socractic method which is very much like what we did. It's all about getting to the root cause of a problem and finding a way to eliminate it. The key thing is the discovery has to come from the person being questioned.