Efficiently use budget

[Guest]

In a video game I play there is a market. On each account (20 total) I have 3 offer slots to buy an item from the market. Each offer slot has a trade limit that resets every 4 hours. The total time I will leave each offer in for is a max of 12 hours. I have a specific budget. Each item (8 available) has a price and a limit of how many can be bought per trade limit of 4 hours. Assuming I buy the max amount every 4 hours, is there an equation that will tell me how many of each item I should try to buy in each offer slot to use up the most amount of money effectively.

Something like

X=budget (known variable)
A,b,c,d,e = items (how many??)
F,g,h,I,j = price (known variable)
K,l,m,n,o = buy limit/4 hours (not sure if this is needed in the equation)

X = (a*f)+(b*g)+(c*h)+(d*l)+(e*j)