I keep an eye out for sales of original cover art (the actual paintings on which novel cover art is based) and got an alert this week on a Ron Lesser oil painting for a late '80's Harlequin Presents, Vampire Lover by Charlotte Lamb. A very subtle, provocative composition, and if it is the original, not a bad price.
Investing in original art can be expensive, but if you have the chance as a published novelist to buy a painting from one of your cover artists, it's a unique way to preserve part of your personal history as a writer. Paintings usually last a lot longer than books, and it's interesting to see how an artist interprets your fiction.
I own two paintings by artist Alan Pollack that were used for the covers of my novels Beyond Varallan and Shockball. When I give talks about how books are published at the local schools, I sometimes bring one of the paintings with me to show the kids how cover art begins. This, btw, always impresses them -- almost as much as the packs of M&Ms I toss to the smart question askers.