The only modules that outputs "number" type is Count and Number Input. So you'll have to use those if you want to use the Math Module. You could create a subpipe that accepts the number input and do the math operation..
You could also extend your pipe by using YQL to do some math operations if you're comfortable with YQL.
well, you can get the dollar amount already, but the equation you want to use , you will need to make a pipe for that literally, starts with a user input number module and then connects to a sequence of math modules till your equations operations are equal within the pipe.
Once done, save the pipe and then, go back to your rent pipe and scroll down to the left bar until you see, in blue letters, my pipes
click my pipes and your math pipe should be accessible , then click the plus arrow for the pipe and it will come upon the operating area. Then, direct the value into the pipe and voila, the end result will be shown