If a private company is developing their own lunar lander, then yes, the software and hardware could be put together relatively inexpensively. I still doubt it could be done for a million dollars simply because of the expense of radiation hardened hardware and the high cost of custom software development. Even private companies would need to put together high quality software or they'd risk losing a very expensive development.

However, I just don't see private companies developing lunar landers for their own use anytime soon. If that's the case, then we're back to government paying for things. That means government standards will be in place. Government software development standards for life-critical and mission-critical systems is very expensive. The government seldom cares what things cost because they know they can go back to the taxpayers to get more money (as witnessed by the Shuttle and ISS).

Mission critical systems like the stock market computer systems are pretty reliable. They have to be - on a typical day the NYSE might trade a billion+ shares of stock, and the other exchanges like NASDAQ and AMX do billions of dollars in business every day. They do have to contend with things like bit errors in data streams but that's different from random bit errors occuring in the heart of the CPU or main memory. Even with all of the best hardware and software they have at their disposal, the stock exchanges aren't immune from computer glitches. Those applications are mission critical but not life critical.

Life critical real-time systems demand the absolute highest quality because errors can result in fatalities. These systems can be things like air traffic control systems, fly-by-wire control systems for airliners (which can have errors in the user interface or in the operational modes), nuclear powerplant control software, etc. Software in these applications requires much more rigorous requirements, coding standards, development practices, testing, etc. All of this stuff costs money.