"If not for you" meaning

Solution 1:

The construction If not for X is an idiomatic phrase in English. The verb to be is normally left out when you use this construction, though if you restore it the full form would be:

If it were not for you, I would be poor.

However, the original version without it were is also correct:

If not for you, I would be poor.

Your version with if there were not you is grammatically incorrect. The construction There is X cannot be used with a pronoun in place of X under most circumstances.

Your example sentence If not for you, whom do I cook the cake for? is also correct, but this is a completely different construction which superficially resembles the one in your question.

Solution 2:

To me, "if not for you" means: If it were not for you to help, I would be poor.

The meaning of "it" is also clear (it means "to help").