HELP!! iWork Numbers formulas

Hi all, need some help on iwork numbers formulas. I am making myself a eBay calculator with Numbers but can't really figure it out how. This is something I want to create:

example 1) $50.01 - $1,000.00 (my closing value will be E3)

8.00% of the initial $50.00, plus 4.50% of the remaining closing value balance ($50.01-$1,000.00)

This is what I figure out =SUM((50*8%)+(E3-50)*4.5%)
which gives me exact correct answer.


example 2) Equal to or Over $1,000.01

8.00% of the initial $50.00, plus 4.50% of the initial $50.01-$1,000.00, plus 1% of the remaining closing value balance ($1,000.01 - closing value)

What will be the formula for this? I have no clue how to type the $50.01 - $1,000.00.

PLEASE HELP!!