Is a full frame camera, such as the Canon Mark 5D, worth the huge difference in price or is it just the lenses that make the difference?
"Worth" is a value judgement that requires criteria. So, the answer is "it depends." Larger sensors cost more than smaller sensors by a fair amount, and the sensor is the most expensive piece of hardware in a camera- but the size of the sensor changes several things- and how much those things are "worth" to you is something you have to decide. For instance, most larger sensors have bigger photosites than smaller sensors. More photons fit in a larger sensor's photosiet than in a smaller one, giving the sensor more to work with for each pixel. That means less amplification of the signal, so less noise. Now, is less noise "worth it?" If you're happy with the noise characteristics of say a T4i, then it's not, but if you're picky, or you shoot things where that's important, then it may be. Same thing with depth of field. Outside of that, "prosumer" level cameras like the 5D series tend to have more manual features, better sealing, and better battery life than consumer bodies like the T4i. A rain cape may be "worth" more than a sealed body to you, or you may find a crop camera with acceptable sealing for the conditions you shoot in for a lower price. Most of the same issues exist in terms of value for the "professional" level cameras vs. the "prosumer" ones, except the price goes up even more and the "worth" tends to go to more and more corner cases.
Only you can decide which features have value to you and what that value is. In real-world terms, for normal prints and computer use all modern DSLRs produce images that are great for most shooting conditions.
In terms of lenses, better lenses produce better results in more extreme conditions (mostly lower light.)
In overall value, I think it's still mostly true that more money spent on good lenses is a better investment than more money spent on better bodies. Also, the lifespan of a lens is a lot longer than that of a body and they hold their value much better. In two-four years, the current "Rebel" model will be better than the latest 5D today, but a great lens will probably be just as good[1] and will have held much more of it's value.
Paul
[1] Unless we keep getting denser sensors, in which case we're going to need better lenses. This happened in the Medium and Large Format world quite a few years ago, and the new "digital" lenses are much better than the others, though many of them held their value pretty well for 50-60 years.