Additionally, you'll also come across the spellings two cents worth (no apostrophe) and two-cents worths (compound form). The apostrophe is not as common as it used to be, at least in the North American English.
two cents' worth =
two cents of worth
From E-mediate Guides:
Bill Bryson, one of the newer breed of grammar doctors, goes dogmatically in the other direction: "Many writers who would never think of omitting the apostrophe in 'a fair day's pay for a fair day's work' often do exactly that when the unit of measure is increased. Consider 'Laker gets further 20 days credit' ( Times headline), 'Mr Taranto, who had 30 years service with the company...' ( The New York Times ). Both 'days' and 'years' should carry an apostrophe. Alternatively we could insert an 'of' after the time elements ('30 days of credit', '19 years of service'). One or the other is necessary." (Bill Bryson, 1984/1987, Dictionary of Troublesome Words ,p177-8).