Sending a level 20 retainer on quick exploration costs 2 ventures (400 seals) and they bring back some junk that gets you 50 GC seals at least. Sending a level 90 retainer costs the same 400 seals but they regularly bring back items worth 2000 seals.

Just like on your main character everything costs less when you're low level since the rewards are also lower, shouldn't venture cost also scale? I've had a huge shortage of seals while leveling my retainers to a point where I could barely keep up and couldn't afford dark matter and other stuff, until they hit a higher level and now I'm swimming in seals just from the stuff they bring back.