It's simple math. First off, we aren't paying for the Ogres every meal, they're paid a salary and they participate in the economy, feed themselves, etc. Oskana isn't necessarily out-eating them, we just have to pay for her every meal (which is like, a cow a day) straight out of pocket.
For the dragon, that's again just math. We know that how much Oskana eats and the cost of it is based off her size. We also know that dragons are larger and eat more than griffins. I'll also note, Oskana's price is more like 4000, it's just reduced thanks to favorable deals and alternative payment methods (like the prestige of feeding your Elector Count's Griffon).
If we actually use Torroar's example price scale of 10 gold per cow/per day, and we say the dragon eats maybe two cows per day, since it's probably gonna eat more than her, that's 20 gold a day, 365 days a year, which comes out to 7,300 gold a year to feed one dragon. Maybe if it eats a bit less, maybe just 1.5 cows a day, that's 5,475 gold a year. Hell, even if I'm wrong and it just eats as much as Oskana, that's another 4000 gold a year, so 8k just to feed two animals. I have also not accounted for the Oskar tax either, because I forgot until this exact moment that we are feeding two griffins already.
