As the first reply said, it varies on your use for them, I'll add two examples for coach.
Two litmus tests for uses I'm used to:
My next hopeful award: Mainland US (East Coast) to Hawaii, usually a $700ish street value round trip at the time of year I want to go, also usually available that time of year for 45,000 miles saver Y, $700/45,000=1.55 cents per mile. Not great, but if you already have the use you want planned, and are looking to minimize cash spend, it saves you $700.
And then, my favorite flight in UA's global network, HNL-MAJ-KWA-KSA-PNI-TKK-GUM on UA154 aka the Micronesian "Island Hopper," is a $1,250ish street value but only 25,000 points saver Y in the off-season (post holidays Jan + Feb), so that's roughly 5 cents per mile.
That alone is more than a 3x spread, but there are probably even more extreme examples out there.