Plugs are quite good these days, as are fuels and fueling maps. I don't know what Yamaha's recommended replacement interval is, but my guess is that plugs are probably cheap if considered as a "per mile" expense, just as oil and tires are also cheap when considered by this metric.
However, these expensive spark plugs can need more frequent replacement if you've modified the ECU and are "tuning."
We ran our student formula car with a YZF R-6 engine, and due to induction modifications that were required by the competition, we had to completely remove the stock fuel and timing components. We tuned on an eddy current chassis dyno, and it was possible for plugs to be harmed during these experiments.
On the shoestring student project budget, the students discovered that if they pulled the expensive plugs, they could clean them off with a propane torch and reuse them.
I would not do this myself, but this was one of the shade tree mechanic ideas that did not seem to cause any specific harm, and it did save some money on an otherwise limited budget.
Another student idea that I saw more than once on some other teams' cars over the years was to remove the stock oil pan and replace it with a more shallow pan, so that the engine could be mounted lower and maintain acceptable ground clearance. The idea was that this might give improved performance on the skid pad or in general cornering.
The stock pan had a very elaborate system of dams, wells, and weirs. The student designed and built replacement pans were nearly always feature free (like a cake pan). These engines were frequently doomed to seizure under heavy use ... so the shade tree logic, i.e. the logic that you know about the engine than the designers, can lead to "learning opportunities."
Thinking about this, another one that I saw a few times was the carbon fiber air box. Some of you know that carbon fiber plates are pretty good under tensile loading, but they're not well suited to compressive loading. An air box in normal use sees interior pressures that are lower than atmospheric, and so the plates must stand up to compression stresses. I'll never forget the time I saw the happiest and saddest moments in one fellow's engineering life. It looked so pretty sitting there before the engine was started. It failed on the first punch of the throttle. Sigh....
It could have been designed to hold up to this type of loading, but in order to keep it as light as possible, they'd made the interior feature free (like the oil pan idea). Sometimes the simplest approach is not the best.
