How to find out which set contains a higher plate percentage?
The best set from 2014 is Furry Creatures (31021) at $0.20 per plate.



Here's one possible way to do this:


Download the listing of all parts in all sets from Rebrickable.
Join this data up with pricing data (perhaps from the Brickset API) to get yourself some pricing information for each set.
Define which parts count as plates.
Write yourself a little program to count the number of plates in each set and divide by the price.


If I use the plates in the plates service pack from 1999 as my plate list, here's what I get for the best price per plate for sets containing at least 150 parts released in 2014:

 id          price_per_plate  plates           us_price
 31021-1     0.201919         99               19.99
 21020-1     0.235802         212              49.99
 31020-1     0.246438         73               17.99
 40182-1     0.249875         80               19.99
 40090-1     0.262895         38               9.99
 40183-1     0.277639         72               19.99
 21107-1     0.286803         122              34.99
 31025-1     0.302955         132              39.99
 40180-1     0.307538         65               19.99
 31024-1     0.319043         94               29.99
 10243-1     0.323866         494              159.99
 31023-1     0.336966         89               29.99
 10242-1     0.352077         284              99.99
 31026-1     0.354291         254              89.99
 60058-1     0.356964         56               19.99
 31019-1     0.356964         56               19.99
 79116-1     0.372287         188              69.99
 75038-1     0.384462         65               24.99
 21117-1     0.402241         174              69.99
 41056-1     0.403065         62               24.99


This is quite expensive if you aren't interested in the other parts as well. You'd be better off buying these on a secondary market like Bricklink or perhaps even directly from Pick-a-Brick.