Signed pieces have a trading value in the marketplace beyond the components' value . Then, if the piece is relatively modern, I check it's retail price in the store . Many fine jewelry manufacturers continue to produce the same design for years. I've seen people bid at auction more than the piece is retail-priced, new in the store. By the same token, sometimes I pay at auction half the retail price .
People are remarkably lazy about doing their homework.
For an older piece, I try to look at recent prices on similar pieces . Such information is only moderately helpful, because auction records don't carry enough specific data. Then, I combine these bits of information and decide how much I am willing to bid. Like picking a stock, the final evalution is part arithmetic and part art . Of course, I hope that other bidders are asleep when my quarry is on the block.