1. Yes, there is a rule. This is an example of inversion, triggered by fronting the sentence with a negative.
In a complex verb phrase which has an auxiliary or modal verb, you simply reverse the position of this and the subject. For example:
You are getting your money's worth ---> Not only are you getting your money's worth...
You will get your money's worth ---> Not only will you get your money's worth...
In simple verb tenses, where the verb phrase only has one word, you need to use 'do' or 'did' to create the inversion (just as you use 'do' or 'did' for negatives and questions).
You get your money's worth --> Not only do you get your money's worth...
You got your money's worth --> Not only did you get your money's worth...
2. No, you can't say "Not only you get your money's worth". This is grammatically impossible - you need to have a verb following directly after 'Not only...'. This can be the verb 'to be' (am, is, were etc) or a modal or auxiliary verb: have, can, must, may, will, would, should.
Check out 'inversion' in your grammar book, and you'll find examples of other phrases which need to be followed by an inversion of the verb and subject, for example 'No longer ...' or 'Never' .