In this instance you would only need to use 'cost'. 'Be cost' is not grammatically correct. So the correct sentence would be: "How much do you reckon it will cost?" Or you could also say: "How much do you reckon it will be?" This sentence is also correct and could be used in the same context but this one does not need the word cost because it is already implied that you are asking about the price of something. If you would like more help on grammar items please feel free to schedule a lesson with me.

Absolutely not. However you can say How much do you reckon it's going to be worth. That's because cost is a verb and worth is a noun.

No. Having 'be' in that sentence is incorrect. You need to remove it.