Medical expense deductions

We have medical expenses that we can either pay in monthly installments or a lump sum.  If we pay the lump sum, it will put us over the 10% of adjusted gross income to be able to deduct.  My question is this, will the amount deducted be a $1 to$1 on our return?  So if we exceed 10% of AGI by $1200, would we be getting $1200 more in our return?  if it is not a $1 to$1, what is the exchange rate?