Mathematical Question

ok guys i need help figuring out something here..
i am trying to incorporate this into a formula in a form


in the retail industry we all determine profitability based on gross margin % and $..

example

if an item cost $250 and you sell it for $599

you can take 250/599 = .417

which tells you that the reverse of that is the actual percentage margin you made.

now look at the logic behind it.

(((250/599)*100)*-1)+100 =58.26%
ideally thats a formula that we dont think of but in our heads thats how we calculate that.

now this formula that we all use (without realizing it) only works up to certain #s. If you start selling items extremely under cost your Gross Margin goes nuts.

Here is an example..

cost = 250
selling price=1

(((250/1)*100)*-1)+100=-24900%

now i am trying to analyze this and make more sense of it. a -24900% margin does not make much sense and this is why.

item A cost 250, sold for 599
item B cost 250, sold for 1


your profit amount for item A =$349
your profit amount for item B= -$249

therefore your Gross Margin $ profit is $100

now when you figure out you Gross Margin % it wont work using the formula since you are averaging (-24900 and 58.26).. it will show a negative margin when i fact you made money on this so its not negative..

any one?