Elasticity question: Finding maximized price.

The consumer demand curve for cases of gadgets is given by $\displaystyle q = (90 - p)^2$, where p is the price per case and q is the demand in weekly sales.

a) If the gadgets cost $30 per case to produce, use calculus to determine the price that should be charged per case to maximize the weekly profit.

I know how to find revenue but the $30 in the equation's throwing me off. On my answer key the answer is $50 per case but I keep coming up with $45. Here's what I am doing:

$\displaystyle R=pq$

$\displaystyle p(90-p)$

$\displaystyle (90p-p^2)$

$\displaystyle (90-2p)$

$\displaystyle -2p+90=0$

$\displaystyle -2p=-90$

$\displaystyle p=45$

I am sure it's something silly considering I am neglecting to incorporate the $30 into the picture but that's what 6 hours of studying at 3am will do to you! :)