I agree that 1 is wrong: it's not the same as 3.
In example 3, It's worth discussing., the word it refers to something previously mentioned, outside of the sentence.
In example 1, it tries to refer to buying this phone, at the end of the same sentence, and it doesn't work.
You could say It's worthwhile buying this phone., which is a way of saying
Buying this phone is worthwhile.
But using worth, that transformation doesn't work:
*Buying this phone is worth.
Adding to the previous answer. Here is a definition of worth:
Merriam-Webster worth
preposition
2: deserving of
well worth the effort
So, worth is a preposition in this use. The M-W definition has an adjective sense, but it is marked as archaic, so it's irrelevant here.
Here is the definition of worthwhile:
Merriam-Webster worthwhile
adjective
1: being worth the time or effort spent
worthwhile preparations
Since worth is a preposition, it requires an object, which it lacks in example 1.
In the examples with worthwhile, as an adjective, it needs a noun or pronoun to apply to.
As to examples 4 and 5, I find 4 understandable - something previously mentioned is linked to the adjectival preposition phrase "worth discussing*.
Example 5 doesn't hold together for me - it wants a last word-
It's worthwhile discussing prices.
If, in example 5, it refers to something outside the sentence, then discussing seems to be hanging there unattached.