Paging Ted...
I've read a number of posts on probe failure on this and other fora.
On a carbureted engine (assuming it is leaned aggressively at 65% power, no LOP ops), does the value of monitoring all four cylinders outweigh the cost of having 4 times as many probes to fail? Is a single EGT/CHT probe on the hottest (#3?) cylinder sufficient?
I don't think probe failure is the major expense associated with engine monitors--initial purchase / installation cost is. Typically, when carbureted engine owners ask if an engine monitor is worthwhile, it's because one of the traditional "paybacks" on an engine monitor for FI engines is LOP operation, which is not guaranteed on a carbureted plane, so they're looking for other reasons to have one.
I installed an Insight G3 on my Lyc O360 last year, and am glad I did. I like the additional data available on my engine in flight, and the detailed data available on the SD card post-flight.
One day, it may help me identify or diagnose an engine issue before it becomes a critical problem in flight.
But until / unless it saves my bacon, it'll be hard to say that there's financial justification to the purchase.
Why spend $$ on Rosen sun visors, when you can tape gray plastic film to the window? Why buy a GTN 650 when you can get a KLN-89B for less?
For most of us recreational flyers, there's no financial justification to anything associated with aviation. You spend the money 'cause you want to. Up to you to decide if an engine monitor is something you want to spend money on.