I was recently reading an article about '15 cancelled PS3 Games' (it was shockingly written, so I'm not going to link it here), and one of the things that struck me was a very promising game called "Eight Days" had been cancelled about 3 years into its development. I researched the subject to find that Shuhei Yoshida, President of Sony WWS, had said that the fact the game didn't have an online mode was "part of the consideration" in cancelling it. To me, this seemed like a really silly move, but it got me thinking about the price of games these days relative to their length.
I've seen a few series such Uncharted, Bioshock & Assassin's Creed add multi-player modes to their later installments which would definitely extent the lifespan of the game if you had any interest in playing them. On the flip side, games like Heavy Rain and Super Mario Galaxy can retail at full price with just a single player mode. And multi-player centric experiences like Call of Duty and Battlefield seem to add a short campaign simply so they can sell the game at full price.
So what's the value of a game to you? Are you happy to pay full price for a 8-12 hour campaign if the quality is good enough or will you wait for it to drop in price? Is a short game with lots of replay value worth the cost? Do you want a game to have some kind of online mode to keep you coming back or does that not interest you? How many hours playtime do you require before you feel the game should give before it has repaid the cost?