Learn English – Difference between: “would be”, “will be”, and “is/are -ing”

Would is conditional/hypothetical. Will is a modal verb used to form the indicative future tense.

In both cases, a) and b) are both correct, and carry a different meaning. When would is used, it expresses a hypothetical situation, so:

What would I do without you?

... asks a question about a hypothetical possibility - that I would be without you. However:

What will I do without you?

... implies that I will in fact be without you at some time in the future, and asks a question about that factual (rather than imagined) eventuality.

That is $200 means the current price is $200. You could tell this to someone who has not expressed any interest in purchasing the item, or you could use it with someone who is actually buying it.

That will be $200 is only used when the person has bought or at least is getting ready to buy the item, and needs to know the price.

That would be $200 is somewhere between the other two - slightly implying that the person you're talking to might buy the item, but not nearly as decisive as will be.