How about creating a tax rate table and then using vlookup to return
the appropriate tax based on purchase price.

Tax Table

price (A1) tax (B1)
00.00 $0.00
350.01 $0.90
550.01 $1.40
850.01 $1.90
1,000.01 $2.15

price value (B8)
tax x

x= vlookup(B8,A2:B6,2,True)

This equation looks up the price in B8 in the tax table defined by
A2:B6 and returns the 2nd column tax amount. Since it range lookup is
set to True, if there is no exact match, vlookup will match against
largest price in column A less than the price entered in B8.

Hope this helps.