I just won 10 more Prince Valiant books at eBay, bringing my library to a total of 27 titles out of the available 50 (the auction I won was actually for 14 books, but I already have four of them). The price per book in this auction was fairly reasonable: only $11.75 each! I've found that buying the books in bulk is the only way to get them at a reasonable price.
Unfortunately, this lot of books was listed in "fair" condition and the picture provided didn't fully show their actual state (at least the spines look to be in decent shape). I guess I'll see what condition they are in when I get them. The beautiful thing about eBay is that I can always turn around and sell them to someone else if I don't like them.
Interestingly enough, I'm still awaiting my order of volume #2 from Amazon. They apparently don't have any, so they have delayed my order for some time (up to November if I recall correctly). But I'm willing to wait - the early volumes are hard to get, especially at the low price of just over $11.00!