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' .

Your alternative is not good English, sadly. "Not only" (used as a limiting adverb, and also other limiting adverbs) at the start of a sentence requires subject verb inversion, and for this we frequently need to insert a "do", or another auxilliary verb. So, in your example "not only" is opening, for emphasis. See https://dictionary.cambridge.org/grammar/british-grammar/word-order-and-focus/not-only-but-also which says 'To add emphasis, we can use "not only" at the beginning of a clause. When we do this, we invert the subject and the verb, and for this "you get" becomes "do you get" since "get you" is used only as a command. This is because : 'do-support (or do-insertion), in English grammar, is the use of the auxiliary verb "do", including its inflected forms "does" and "did", to form negated clauses and questions as well as other constructions in which subject–auxiliary inversion is required.' This explanation is from " https://en.wikipedia.org/wiki/Do-support I hope it helps :)

The word 'do' can't be eliminated here. You could replace 'do' with 'will' here and say, "Not only will you get your money's worth, but ...". This sentence has the same meaning (or very close). Perhaps this helps you understand the meaning of 'do' in this context ?

I believe the verb is used to describe the act of you getting the money. So for example if you are not getting the money. The sentence " Not only you not get the money" would not make sense. The correct sencences are, 1) Not only do you get the money... 2) Not only do you not get the money..., ( opposite)