This sentence is just an example of one of the complexieties of the English Language. While it can be parsed as the relative clause “which is not worth grabbing”, I will take a different approach because the relative which clause can refer to a whole sentence (as suggested by Will Crawford’s brief answer), ranging from basic to very complex:
Chris did really well in his exams, which is quite a surprise.
My friends were all hiding in my apartment, which isn't what I'd expected.
She's studying to become a doctor, which is difficult.
Source: https://www.ef.co.uk/english-resources/english-grammar/non-defining-relative-clauses/
I think the other thing that was really good about it as well was that everybody worked really hard and helped tidy up at the end, which I hadn’t expected at all.
Source: https://dictionary.cambridge.org/grammar/british-grammar/relative-clauses-referring-to-a-whole-sentence
The OP’s example is a non-defining relative which clause.
Non-defining relative clauses are composed of a relative pronoun, a verb, and optional other elements such as the subject or object of the verb. Commas or parentheses are always used to separate non-defining relative clauses from the rest of the sentence.
Answer
In the way the sentence is contructed, IT IS UNGRAMMATICAL as the object 'that clause' is missing a verb which in this case is “means”:
“You know all you’re getting off it then is maybe the CD-ROM, which surely means that’s not worth grabbing”.
As mentioned in EF’s quote a non-defining relative clause must be separated with a comma, but Cambridge notes:
We often use these clauses in informal speaking to express an opinion or evaluation [...]
In speaking we sometimes pause before these clauses:
She just lives six doors away, [pause] which is very handy.
This is a perfectly possible explanation as to why a comma was not included as in speech it is normally represented through a pause.
In everyday speech, it is therefore possible to create a number of different non-defining relative clauses which can just be as ambigious:
- The girl who is cooking the fish which father caught is my sister.
- The girl who is cooking the fish that father caught is my sister.
- The girl who is cooking the fish father caught is my sister.
- The girl cooking the fish father caught is my sister.
- The girl cooking the fish that father caught is my sister.
- The girl cooking the fish which father caught is my sister.
- The girl that is cooking the fish that father caught is my sister.
- The girl that is cooking the fish which father caught is my sister.
- The girl that is cooking the fish father caught is my sister.
While it is clear to some that five is the most sensible, all 9 sentences are perfectly grammatical but subject to some better stylistic choices.
Other possible resources to read: Edwin’s answer on Double relative clause.