From last week’s New York Times:
“Weather apps are two a penny, but I’ve used one more than any other this year: Yahoo Weather.”
The Oxford English Dictionary’s definition for “two a penny” (“ten a penny” is a variant): “plentiful and consequently of little value, commonplace; easily obtainable or available; occurring frequently.” The OED quotes a 1948 novel by Neville Shute: “In Hollywood beauties were two a penny, and it was years before she got an inkling what it was that differentiated her from all the stand-ins and walkers-on.”
Before reading the quote in the Times, I was unfamiliar with “two a penny.” Searching the the newspaper’s archives, I see the last time it was used was in a 2005 review of a play by Alan Ayckbourn, which was kind of appropriate, in view of Ayckbourn’s nationality. Given that there is an apparently completely synonymous American cliche–“a dime a dozen”–I don’t expect to encounter “two a penny” here again. But as Fats Waller immortally said, one never knows, do one?
News flash: Mere minutes after this was posted, the following came over Twitter, from Kit Eaton, who wrote the line about Yahoo Weather:
notoneoffbritishisms.com 
When I read the phrase in question here, my mind went immediately to the opening line of my favorite song from the 1968 musical, /Oliver/, “Who Will Buy”: “Who will buy my sweet red roses? Two blooms for a penny,” sung by the rose seller in an ensemble chorus. I suspect composer Lionel Bart strung out the phrase to get it to scan musically. https://youtu.be/OntYSs5DsJ0
‘In for a penny, in for a pound’.
‘Penny for the guy’.
‘Penny wise – pound foolish.’
‘Penny for them’.
‘Spend a penny’.
In pre-Decimal (1971) and early Decimal UK there were halfpenny (pronounced hape’ny) coins. The amount, two a penny, was relatively common and had a recognized, if small, cash value. To stress a valueless amount, the expression therefore would have to be “ten a penny”.
Despite the British credentials of Messrs. Shute and Eaton, “two a penny” sounds like an Australian / American corruption.
The American “dime a dozen” alliterates well but “ten a penny” has better consonance.
I also remember it as “ten a penny”. Maybe “two a penny” is inflation?
I (English) am familiar with ‘two a penny’, but no other numbers.
I suspect the familiarity of that form may be related to the (18th-century) nursery rhyme with the refrain “one a penny, two a penny, hot cross buns!” (See https://en.wikipedia.org/wiki/Hot_Cross_Buns )
That’s exactly what I thought of when I saw the phrase.
http://news.bbc.co.uk/1/hi/magazine/8598312.stm
Where I’m from “ten a penny” is quite a common saying, I’m in Lancashire.
I’m with Bendy. Up North: ‘ten a penny’!
I’m in Southampton and use it regularly,
It is all down to the relative value of the currency when they were popular.Before the UK switched to decimal numbers for the pound there used to be 20 shillings to the pound. Each shilling was worth 12 pennies.When decimalisation took place it was probably to disguise the falling purchasing power of the smaller coins.The value of coin was 1/2,1,3d,6d,1shilling,2shilling,2shilling and 6 pence other coin values were generally for collectors and not in general circulation.Pre 1960`s 1 penny was worth a lot of money therefore in Victorian times it would have been a fortune and that is probably where the 10 a penny came from.
Not sure when you were born, but I’m 1954 vintage (so, just missed farthings), and 1d was not, I assure you, a lot of money. A call from a public phone box was 4d. A portion of chips was 3d from the chippy, and I remember the price going up to 4d. Some sweets (notably, midget gems and sports mixtures) were four for 1d.