A suggestion would be to have 2 tables:
Table 1: Purchases
Table 2: Items
Set up 1 relationship:
1 purchase to many Items
In the Items table, create a field called cost.
In the relationship table, create a summary field called total cost.
In the purchases table, create a formula-number field called markup with the following formula:
If(([Total cost]/50)<1,0,
If(([Total cost]/50)>=1,[Total cost]/50))
the "[Total cost]/50))" section would be based on the what you'd prefer to increment by.
for example:
If you'd prefer to have a markup of 50% for every 50 dollars, it would be ([Total cost]/50)*.5
You'd use this instead:
If(([Total cost]/50)<1,0,
If(([Total cost]/50)>=1,([Total cost]/50)*.5))
In the purchase table, create a formula-number field called final price with the following formula:
[markup]+[Total cost]
Now, you'd be able to add multiple items to a purchase and apply a markup to the total amount when the total amount reaches a limit.
Without the 40 lined case formula, you'd loose the ability to control each level individually.