The first calculators were abacuses, followed by the slide rule (invented by William Oughtred in 1622), and then mechanical calculators in the 1900s. The first electronic calculators emerged in the 1960s, but were large and heavy because they required many transistors. It wasn't until the 1970s that integrated circuits made pocket electronic calculators possible.
Most vintage pockets electronic calculators are a dime a dozen, with few exceptions (an original 1975 HP-25 programmable calculator in great condition might be worth about $100 or so). A richer vein to explore is mechanical calculators, which were produced in large volumes for business users, beginning in the 1880s with the now collectible Comptometers and Burroughs adding calculators.
True vintage calculator aficionados will want a cylindrical mechanical calculator made by Curta in the late 1940s (Type I) or mid-1950s (Type II). Designed in a WW2 concentration camp by entrepreneur Curt Herzstark and made in Liechtenstein, only about 140,000 of these compact devices were produced. They were the first truly hand-held calculator, utilizing a stepped-gear calculating mechanism. Popular at the time with land surveyors and sports car rally spectators, today they command close to $1,000 in good condition.
On the other end of the scale, if you’re just looking to show off a vintage calculating device without spending big bucks, there's the ‘Little Professor’ calculator game, introduced in 1976 by Texas Instruments, complete with red LEDs. Instead of typing in problems, you type in the answers… but for $25 bucks, who cares?