How much is a pie? thanks.

"Pi", written as π - is an irrational number.

π≈3.14

If it's really fancy, like with custard inside and whipped cream on top,
it might cost as much as $10 depending on where you buy it.

If you're talking about the greek letter 'pi'  ( π ) that's used in math, then
there's no exact answer, because the exact value of that number can never
be written on paper with digits.

It starts out with  3.14159 26535 89792  and it just keeps going and never ends.

If you need to work with  pi  in a math problem, then your answer can never
be exactly exact, but you can get as close to it as you want to by using
more digits in the decimal.  3.14  is usually good enough, but supercomputers
have been used to calculate something like 4 trillion digits of  pi  after the decimal
point, and it still never ends !

If you use either  3.14  or  ' 22 / 7 '  for  pi, then your answer is only about 0.05% 
wrong, and that's pretty good !