There is, sort of. It's easy to look at the interior of, say, a 737 and conclude there are only two or three "classes" (economy and first, and possibly some sort of premium-rate economy with extra legroom), but fare-wise there's enormous variation.
On -- to take an example legacy carrier -- United, the "economy" cabin is actually made up of fourteen different fare classes. Deciding how many seats to offer in each fare class on a given instance of a given flight on a given day is a stupendously complex task which largely gets hidden from the customer (for completeness' sake, "first class" and "business class", on planes which have them, have three additional fare classes each).
A big part of it is that there's more to it than just the seat, which is obvious since all those different "economy" fare classes end up in the same or very similar physical things-to-sit-on. The number of factors, though, is frighteningly large.
For example, your origin and ultimate destination factor into the fare, as do whether it's one-way or round-trip, and how far in advance you booked, and whether you wanted a refundable ticket, and whether it was booked by a corporate travel department on your behalf, and whether it was a seat offered to third-party travel-booking and search sites, and what day of week and time of day the flight is operating, and whether it's a seasonal offer to a popular vacation destination, and... that's the tip of the iceberg.
1: http://www.dailymail.co.uk/news/article-2329405/Virgin-Ameri... 2: http://www.bizjournals.com/sanfrancisco/news/2013/08/07/eyei...
I was looking into flights to Japan, and the Premium Economy tickets are literally double the regular economy tickets.
Many domestic flights have Premium Economy for less than double the price, but it's still expensive enough and the benefits not great enough (generally only a couple of inches of legroom--no change in seat width, no better service, etc) to generally not be worth it unless you are flying cross country.
When United and Continental merged and began adding E+ to the Continental fleet, most aircraft were completed pretty quickly since that change can be made during overnight maintenance at the hubs.