When my print rep talks about five-colour printing, what does he mean? I know you have cyan, magenta, yellow and black - but what's the fifth?
In Offset printing the fifth colour is an additional spot colour, which could be anything (of course depending on the printer/printer services). I've yet to hear what is the single most common "fifth colour".
Why? For example in (now discontinued) hexachrome (CMYKOG, yes 6 colours) printing process orange and green were added to achieve wider colour gamut and better colour reproduciton. Also, it could be unmixable colour such as Pantone metallics or fluorescent to add more effect, or some secret mixture which would be printed onto your paper money for added security.
And as Philip Reagan commented, the fifth colour is usually a crucial colour for the corporate/brand image, such as the "Coca-cola red", "IBM blue" or "Ferrari red"; when the colour is best to be consistent (and not dependent on the mixing).
The most accurate answer, with cost estimates, you can only get from your print rep.
In addition to koiyu's excellent answer, a fifth color might be a "double hit" of a color which is being printed in CMYK, to make it pop. For example, Pantone 186 red might print a little muddy in process, so a fifth plate is added of the same Pantone 186 in spot, to make the red brighter.
I've also heard of the fifth "color" used to refer to a varnish coat, because it's a fifth plate.