Two/ten a penny
Definition 1 of 1
Pro Tip 1/3Understanding Value
Phrase implies items are so plentiful, they're valued less because they are easy to find. 
With every house having a computer, they are now ten a penny.
Pro Tip 2/3Culture-Specific
Primarily used in UK English, may confuse speakers of other English varieties where the phrase isn't common. 
He was surprised when his American friend didn't understand 'two a penny'.
Pro Tip 3/3Not Literal
It’s a figurative term, not used for actual pricing. Teaches about idiomatic expression. 
Mobile apps these days are so common, they're practically ten a penny.