When buying Original Art, you should become familiar with the different types of pages available and the common terms used to describe them. We'll illustrate each term with original art samples from Bob's collection.
Cover - Pretty self-explanatory, the actual cover art from a comic book. Be careful, not all full-page images are covers, and not all with cover dress were actually used for a published book, sometimes a cover is rejected by editorial & needs to be redrawn in part or in total.
A single image that takes up the full page. Sometimes if there is a small inset panel it can still be considered a splash page, but be careful of people describing a half-splash as a splash page.
A single image filling 2 side-by-side pages. Kind of a double-sized splash page.
A double-page spread that does not contain a single image. Half is a single splash image that is augmented by a number of other single panel illustrations.
A single page divided into to 2 half-page images.
A single image takes up half the page.
Pretty much everything else!
Sometimes artists will provide a rough breakdown of the script in thumbnail form. Usually they are less detailed than the final pencils and are consequently cheaper than the same pages. However some artist provide very nice breakdowns which can be a great way to get artwork at a good value.
As part of the color process at a certain time in comics, there are color copies of artwork noting the different colors to be used for the printer. They are generally no longer used in modern color processing. It seems like, generally speaking, there has not been significant appreciation of price in this market and pages can be found from $20 - $40 dollars. These are pretty much obsolete for new comics, which are directly colored on the computer by the colorist using a tool such as PhotoShop.
Scripts are sometimes reproduced as an extra feature in a special issue of a comic or a collected edition, but if you meet a writer at a convention, sometimes they will be selling copies of their scripts. These are nice keepsakes but don't have a lot of collectible value.
Artists will often accept commission requests (for money) from fans. These commissions can range from simple sketches to fully painted over-sized artwork. They can also be created by the artist for their own amusement/edification. These can range from quick (and very loose) sketches that take a few minutes to "con sketches" that may take 20 minutes to an hour to "full commissions" that may take a day or more to complete. Prices on these vary based on the artist and the amount of effort they put into the art. They can range from free for a quick 1 minute sketch to something in the $40-$100 range for a con sketch to $100s or $1000s for a full commission. You can see more of these on our sketch blog.
Quick Sketch: Scarlet Witch by George Perez
Con Sketch: Zatanna by George Perez
Copies of a piece of artwork, sometimes limited in print run. For digital artwork, sometimes you can get a high quality print copy certified by the artist to be the only such copy but unsure of the market for that. Some of the more popular methods for creating high-quality prints are described below. When the reproduction method is not apparent, it may be best to ask about the printing method that was used. In some cases, a signed lithograph can have more collectible value than reproductions made with other methods, but the print quality of lithographs can vary. Another key factor is the quality/type of paper used.