Basic Calculator

Befunge - 37 x 5 = 185 38 x 3 = 114 characters

This is limited to integer numbers as Befunge has no floating point support.

&v      /& _ #`&# "-"$# -#<          v
 >~:0`!#v_:" "`! #v_:","`#^_"*"`#v_&*>
 ^      ># $ .# @#<              >&+ 

Explanation

The biggest distinguishing feature of Befunge is that instead of being a linear set of instructions like most languages; it is a 2d grid of single character instructions, where control can flow in any direction.

The first & simply inputs the first number. The v and > then redirect control to the main path on the second row.

~:0`!#v_

This inputs a character (~), duplicates it (:), pushes zero onto the stack (0), pops the top two elements and determines if the second is greater than the first (` I'm surprised you can't use ``` to get code backticks.), inverts the truthiness of the top element (!), then goes right if it is zero, down otherwise (#v_).

Basically it's checking whether the input is -1 representing no more input.

># $ .# @

If the input was -1 then the duplicated input value is discarded ($), the top of the stack is output as an integer (.) and the program is halted (@).

:" "`! #v_

Otherwise a similar process is repeated to determine if the input is less than or equal to a space. If it is a space then control goes down, otherwise control heads right.

^      ># $ .# @#<

If it is a space then it's redirected left (<); the program halt (@), output (.) and right redirection (>) are all skipped using #; but the discard is executed to remove the space from the stack. Finally it's redirected up to begin the next execution (^).

:","`#^_

If it wasn't a space the same process is used to split on if it is in [+, *] or in [-, \] going right and up respectively.

 >~                         "*"`#v_&*>
 ^                               >&+

For [+, *] it is again split to determine whether it is a + or a *. If + it is directed down then the next number is input (&) and they are added (+), the control then wraps around and is redirected up to the main path for the next character. If * then it inputs (&) and multiplies (*) then directly wraps around.

/& _ #`&# "-"$# -#<

For [-, \] it starts on the right heading left. The #'s skip the character after them so the initial path is "-"`_ which simply determines if it is - or /. If it is / then it continues left to input (&) and divide (/). If it is - then it heads right, again skipping characters so that it executes &"-"$- resulting in the number being input (&) the - character being pushed onto the stack then discarded ("-"$) and then the subtraction being calculated (-). The control is then redirected back to the main path.

Ruby - 74 69 67 65 characters

a=0
("+ "+$<.read).split.each_slice 2{|b,c|a=a.send b,c.to_f}
p a

Python 3, 105 bytes

Manages the four basic operations, but it only costs 5 characters each to add ^ or %.

f=float
x,*l=input().split()
while l:o,y,*l=l;x,y=f(x),f(y);x=[x+y,x-y,x*y,x/y]['+-*/'.find(o)]
print(x)

Precedence of operations is left to right.