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.