That was my doing a little while back to bring a bit more relative balance to improvement costs (if a path costs 1
, then the cheapest improvements ought to cost more than that; 3
would be reasonable as well, but not 0). The values are not perfect ofc, but it's the general principle (and increments of ~5 are easy to assign + players understand).
The thing I'd like to point out in your comment is "
For C2C in early Preh era". In early prehistoric era - before tribalism - you're lucky to have a city with 4 pop, which means you're really only needing 4-5 improvements (unless you play with a high resource density). At 5
per improvement, you're looking at only 25
or so; this is on par with upgrading a single unit right around tribalism, and should be an amount easily obtainable (single tribal village, lowering tech rate for a few turns, etc). One should not be covering every single owned tile with seasonal camps in literally the very first era of the game; that should only done when you have excess gold/production to support it.
Later in prehistoric once you start to get multiple cities, this cost will of course grow - but that's part of the check to overly rapid expansion, and is when you start to get more dedicated
buildings anyway. It makes it more of an interesting choice how to spend gold - tech, maintenance for cities, or improving infrastructure - rather than just maxing out tech and pouring excess into growing # of cities.