While you cannot exactly get diamond rings for cheap, the money you save from a clever purchase can take care of a lot of your other needs, which means you should pay attention to a few small things when making a purchase.
Carat relates to weight and not size. A diamond of higher carat will not always look bigger, because the weight can be distributed in a number of directions. A deep pavilion or a thick girdle can make the stone seem smaller than otherwise. Higher carat diamonds cost more. If apparent size is what you are after, then choosing a suitable cut goes much farther in easing the strain on your wallet.
Color is a vital aspect when buying a diamond. With a white color stone chosen, the next step is deciding the setting. Mounting on a yellow gold ring, for instance, will cause that color to be reflected in the stone, which means a lower color than D is sufficient. Even I is fine most of the time, although if the metal is platinum or white gold, you will want to keep it above G.