In a recent game I restarted, I unlocked Scout Starship IV right at the start, and gained long-term strategic advantages from complete early scounting. I discovered that a good third of the galaxy contained only two useful planets, which I grabbed early to divert AI reinforcements. I also discovered I'd been going for the "wrong" advanced factory, as the other one had more useful golems and fabs along the way. Afterwards the starships were convenient for quickly assessing threat on planets where I didn't have lurking scouts, as well as quick sniper protection when I'd forgotten to hold ctrl when selecting my ships
.
Assuming that unlocking Scout Starship IVs will save you unlocking Mark III scouts, and that you'll still unlock ordinary Mark II scouts anyway, their knowledge cost is effectively 3,500. Although with Spirecraft enabled, you can achieve complete scouting using a couple of Spirecraft
Jumpships (not Scouts).