Diamonds are not only priced by size, they are also priced by quality. Meaning, if it's not a certified diamond of a certain quality, it can be huge, but still pretty cheap. (well, in comparison anyway) Diamonds are "graded" on the 4 Cs--and Carat size is only one "C". It also includes things like how many inclusions ("Clarity" think of these as "faults"--some are visible to the naked eye, some are not) and Color (or really, lack of color/whiteness). The last C is Cut.
For example, you could get a non-certified 1 carat diamond with a low grade for much less $$ than a certified diamond of the same size that was graded G-VVS-1 or better.
Check out this website, it has a really good explanation of the 4-Cs and also you can see how much various graded/certifications are priced.
http://www.bluenile.com/diamonds_education.asp?track=row_mod7&elem=miniheroHdr
Just because someone has a "big" diamond, doesn't mean they paid a lot for it. 1 carat solitaires could range in price from the low single digit thousands, to well over 20,000 (actually the sky is the limit, lol) depending on its grade/certification.