All of the persisting foods give 30 min buffs…
The key here is to calculate what is cost effective. Bear with me for a minute or two while I take you through some theoretical math 
We all know persisting foods extend the lifespan of our tools. Many of us know (thanks to my earlier equation @boundmore) that your durability reduction skill has a large effect on how (and how well) these foods work. What most of us don’t really have a good grasp of, is what the actual value of the foods are to us as consumers.
I’m going to try and derive a theoretical equation here where you can input your specific variables and see what’s worth what at the end if the day.
Lets start by assuming a tool (gem or not) has b base durability. Your durability reduction - as shown on your character stats page - is d%. @boundmore summed it up nicely for us:
Soooo… This tells us how many actions or “swings” we get out of a tool based on our durability stats modifier. Before food buffs, for untrained characters, d= 0 and for fully trained characters d=40.
When we bring food into the mix, this increases d according to food quality. Lets add a variable “f” here, food buff.
The total equation would read
m’ = b/(1 - [d+f]/100) if d is taken as your skill reduction only.
Remember now that this modified durability only lasts as long as your food does, being 30 minutes.
To calculate the marginal value of your food, you must be able to determine the durability saving in those 30 mins compared to the expected durability loss without food buffs during those 30 mins.
This depends on the number of swings you can perform in 30mins, which in turn depends on both your agility with tools and the swing speed of the tool itself (iron vs copper for example, I’ll refine these numbers further in the future as I’m on holiday at the moment).
For argument’s sake, lets assume you perform s swings per minute. In 30 minutes, you perform 30 x s (“S”) swings.
To work out how much tool you have saved, we need to calculate the difference between unbuffed and buffed usage over these 30mins.
Unbuffed durability loss fraction = S / m
= S / {b/(1 - d/100)}
= S x {(1-d/100)/b}
=(S - Sd/100) / b
Buffed durability loss fraction = S / m’
= S / {b/(1 - [d+f]/100)}
= (S - S(d+f)/100) /b
Unbuffed - buffed loss
= ({S-Sd/100} - {S - Sd/100 - Sf/100})/b
= (S - Sd/100 - S +Sd/100 + Sf/100) /b
= Sf/100b
If my calculations are correct (and granted they were done on my mobile screen, please check) this means that the amount of tool you save in 30 minutes is directly related to swing speed and food buff strength (S,f), and inversely related to your items original durability (b)
It also means that you save Sf/100b % of your tool by buying and using foods. So the food’s worth is Sf/100b of your tool price. If you’re paying more, you’d be better off buying more tools instead of food. If you’re paying less, the food is cheaper than extra tools.
Nb- this assumes your tool lasts longer than 30mins without foods,
This doesn’t show the increase in durability, it just gives a percentage of tool saved - to calculate the added “value” due to persisting foods.
Please check me!