Ideas are cheap, implementation is costly.
I’ve been using this and variants of this since the mid 2000s.
It conveys a fundamental point for me both about the things we create: products, research projects, communities, as well as about life more generally: it is simple to imagine playing the piano like Glenn Gould but takes years to even try, it is easy to imagine concentrating on your breath for 30 minutes but years of practice to do.
There are, of course, some caveats. Good ideas are not necessarily cheap, especially in areas like academic research. Good here usually means something like an idea which is reasonably original (no immediate prior art) and with a good chance of success. (Thanks to my friend Liam for emphasizing this point).