Simply, as soon as the number of signed books becomes finite, the price immediately jumps. A book's price can vary according to region. A particular title may be more popular in the country's mid-section than on the coasts, for example. Or an author may sign extensively in a certain area but doesn't travel often. Region is becoming less of a factor with the rise of computerized book networks and Internet shopping, but most trading still goes on in the local bookshops.
Autographed books come in a couple of varieties.
Signed limited editions are books that are made to be collected. Within a limited printing, the lower the number on a copy the better. But a good limited will be made of better quality materials and may contain art and other features unique to the edition. Some come in a slipcase. Limited editions are not really intended for reading and most will never leave their shrink-wrapping.