Not all deals need to be "the cheapest". There are some excellent deals around for the Premium Economy cabin (where it exists) on many routes. Thing is, you do have to book well in advance and they only really work if, once booked, you do not change anything.
The fare rules often say you can change your flights, but in practice, it is not cost effective to do so. The reason behind this, is that the cheap Premium/ Business Class offers rely on a (very) limited number of seats being available at a given discounted price. Chances are, when you wish to change, only full price seats are likely to be available. Do read the fare rules carefully, though. Some of the heavily discounted Premium (and Business) class tickets do not allow ANY changes (Oh! And none allow refunds!).
Usually, if one can change, one can only change within the cabin selected; so you cannot purchase a discounted premium economy ticket and then downgrade, if changing your Premium ticket is too expensive. The same, incidentally, applies to the discounted business class tickets. As I said, buying discounted Premium and Business class tickets only works if you are 110% sure of your itinerary. The flights need to be booked well in advance (at least 21 days, usually) and availability certainly varies from route to route and season to season.
For example, I recently dealt with a booking for a return flight from London Airport to Washington, DC and the clients were travelling in March. Here, we are dealing with direct flights - I know that better prices may often involve more indirect routes - but in this instance, we are looking at British Airways direct. The best fare offered by BA in ECONOMY (aka "World Traveler" or "Coach") came to £620.87. The fare in PREMIUM ECONOMY (aka "World Traveler Plus") is £838.67.
To get a better perspective, the fully flexible fare in Premium Economy is £2557.87.
Either way you look at it, for an extra £217.80 you are gaining significant extra comfort, a better meal, an extra hold bag (which is alone worth £65!) and a dedicated check-in. The extra money you are spending is, in effect, on you the traveler rather than having to pay for extra items, yet still to be shoe-horned into a seat somewhere down the back.
In the case of British Airways and Virgin Atlantic certainly, the Premium Cabin is, to my mind, well worth the extra money. With other airlines, you do need to check on what, exactly, is being offered. Some airlines simply take out a few rows at the front of the economy cabin and give them a fancy name - there is little material physical benefit for the traveler. Airlines are slowly waking up to the idea of a Premium Cabin; as business class grows more like First, slowly, Premium is becoming the new business. The point is that an extra slice of service, with Premium, is an attainable goal and what you pay extra, is of direct benefit to your comfort.