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Those initial scientific calculators were quite expensive (I think the HP-35 was $395 - in 1972).

I was fortunate. When I came along a few years later the HP-11C was on sale at the Virginia Tech University bookstore for about $85. I bought one in 1982 and I still have it. It's got quite a bit of wear, and it once got dropped in to a sewage pump station during a start-up test, but it still works as well today as it did 36 years ago.

By the way, I also have an HP-35 I picked up a few years back. It now has a burned out element on its LED display so I'll need to tear in to it some time and replace the display module.

If you have any interest today in HP calculators, there is an excellent HP-41 emulator available for smart phones. The app functions as an HP-41CX (the crown jewel of calculators - the one space shuttle astronauts carried as a back-up navigation computer). It includes emulators for just about every plug-in module HP ever made for that series of calculators. I think it costs about $10-$15 for the full version. It's one of the few phone apps I have ever paid real money for, and it was well worth it.

I have the HP calculator on my Android phone. Just a natural to use as any HP calculator. My college graduation gift was an HP25. I wrote my first program on it to replace tables in the Schaum's Genetics book. I carried the HP16C while teaching my embedded systems (6800/8085) course from 1980 to 1983. All hand-coded assembler. Having the 2's comp arithmetic support was helpful for long relative branch back calcs. The HP12C serves as my financial tool. I did finally cough up for a top of the line HP graphing/programmable calculator. Very nice. I keep a lower end one at hand when writing the aeronautical engineering calculations. Sufficient for that kind of work.