The 1970s was the boom time for calculator manufaturers. The machines were, at least for their first few years, something of a novelty and staggeringly expensive - often costing a week's wages or more - so there were profits to be had.

Calculators also provided a path of natural progression in to the manufacturing of computers (a few years later) for several companies, including Hewlett-Packard. Commodore was another that followed this evolutionary path. They came from being an office equipment manufacture, moved on to adding machines and then into calculators in 1970, before launching the world's first personal computer (the PET) in 1977. Hewlett-Packard, on the other hand, came from a background of test equipment, instrumentation and medial electronics and saw calculators as something their customers could use instead of slide rules. Both companies managed to ride out the calculator wars of the middle 1970s, a financial bloodbath triggered by Texas Instruments' move in to the industry. TI had been a supplier of calculator logic chips for years, but started producing their own calculators and selling complete machines for less than they sold the chips to everyone else - few companies survived, but HP was one and they continue to manufacture calculators to this day (2013).

Meanwhile, the advert features a good selection of HP's 1976 line of calculators, from the $80 HP-21 scientific, right up to the staggeringly-expensive HP-97 - yours for $750. That's over £3,000 in 2013 money - more than a reasonable second-hand car. At least the HP-97 offer "about 3.4 times the programming power" of the older HP-65, so it's totally worth it then.