I think it functions as an adjective, though Eliot is right in pointing out that 'value' is usually a noun.
Does the phrase describe the noun? Then it's adjectival (ie. it's not "an adjective" - a single descriptive word - but the phrase functions as one).
So the guesthouse becomes a "good value" guesthouse. However, "good guesthouse" is very vague, and "value guesthouse" makes little sense. the adjectival phrase can't be cut down.