Believe what it does is counts substats (multiplied by prime and non-prime values) and adventures multiplied by VALUE_OF_ADVENTURE, with that value divided by fullness/booze/spleen. So VOA is the total value you're giving to an adventure... if you're making 2.5k meat per adventure, don't try to spend 2.5k for your adventures or you'll just break even, on average.
Anyways, EatDrink will take whatever item has the most 'value' left after it's done.
Math example time:
First VOA: 2500.
Item 1 costs 4000 meat for 1 fullness and 4 adventures and no substats (to make it easier). 4*2500=10000 value, minus the 4000 meat cost, giving it a total 'value' per fullness of 6000.
Item 2 costs 1200 meat for 1 fullness and 3 adventures and no substats. 3*2500=7500 value, minus the 1200 meat cost, giving it a total 'value' per fullness of 6300.
Item 3 costs 3000 meat for 2 fullness and 7 adventures and no substats. 7*2500=17500 value, minus the 3000 meat cost, giving us 14500, divided by 2 fullness for a total 'value' per fullness of 7250.
If these were our only options for 15 fullness and no STEPMEAT (-1), EatDrink would eat 7 of Item 3 (7*3000) and 1 of Item 2 (1*1200), for a total of 22200 meat. For 22.2k meat, we'd get 52 adventures. If we actually make 2500 meat per adventure, we'd end up making a total of 130,000 meat from the farming, minus the 22.2, for a total profit of 107.8k meat.
Second VOA: 4000. Same items.
Item 1 costs 4000 meat for 1 fullness and 4 adventures and no substats. 4*4000=16000 value, minus the 4000 meat cost, giving it a total 'value' per fullness of 12000.
Item 2 costs 1200 meat for 1 fullness and 3 adventures and no substats. 3*4000=12000 value, minus the 1200 meat cost, giving it a total 'value' per fullness of 10800.
Item 3 costs 3000 meat for 2 fullness and 7 adventures and no substats. 7*4000=28000 value, minus the 3000 meat cost, giving us 25000, divided by 2 fullness for a total 'value' per fullness of 12500.
If these were our only options for 15 fullness and no STEPMEAT (-1), EatDrink would eat 7 of Item 3 (7*3000) and 1 of Item 1 (1*4000), for a total of 25000 meat. For 25k meat, we'd get 53 adventures. If we actually make 4000 meat per adventure, we'd end up making a total of 212,000 meat from the farming, minus the 25k, for a total profit of 187k meat.
Final VOA: 10000. Same items.
Item 1 costs 4000 meat for 1 fullness and 4 adventures and no substats. 4*10000=40000 value, minus the 4000 meat cost, giving it a total 'value' per fullness of 36000.
Item 2 costs 1200 meat for 1 fullness and 3 adventures and no substats. 3*10000=30000 value, minus the 1200 meat cost, giving it a total 'value' per fullness of 28800.
Item 3 costs 3000 meat for 2 fullness and 7 adventures and no substats. 7*10000=70000 value, minus the 3000 meat cost, giving us 67000, divided by 2 fullness for a total 'value' per fullness of 33500.
If these were our only options for 15 fullness and no STEPMEAT (-1), EatDrink would eat 15 of Item 1 (15*4000), for a total of 60000 meat. For 60k meat, we'd get 60 adventures. If we actually make 10000 meat per adventure, we'd end up making a total of 600,000 meat from the farming, minus the 60k, for a total profit of 540k meat.
Using the VoA from example 3 and making the actual profit from number 2 would be 240k (60*4000) minus 60k cost for a total profit of 180k. Using VoA from example 3 and actual profit of number 1 would be 150k (60*2500) minus 60k cost for a total profit of 90k. 180k is less than 187k and 90k is less than 107.8k... so basically, math shows us that knowing how much profit you make will help EatDrink pick the best items to help you actually gain meat instead of wasting it.
Anyways, hope that helps. It's a short example with just 3 hypothetical food items, but that's basically the way the comparison works. You tell it how much to value an adventure, and it checks what will be the best options to help you profit.
Edit: Also, my best farming VoA lately has averaged about 4400 meat per adventure... but that's not counting the buffs, which probably drag it down by a few hundred.