I'm kinda confused by your problem, if you have a picture to go along with it that would be great. I dont know the exacts of your problem but I know what type of problem this is: Optimization/Minimalization. Heres what you need to do:
1) Set up two equations with the things you know in it, and the exact amount of piping of a certain price as x and y variables. Make sure one of the equations is for how much the total cost will be, the other will probably be for something like how much piping they have for something of a more geometric meaning. Make sure both equations are in terms of x and y, whatever x and y may be (probably the amount of piping for 230 and for 260)
2) In the equation that isnt giving the total cost, move the variables around so one variable is on one side all by itself. This is equating one variable in terms of another, so you might have or something like that, thats just an example.
3) Take this expression for your one variable and plug it into the cost equation, you should realize that now your cost equation is all in terms of one variable.
4) Graph the cost equation now in one variable on your calulator, and than just look for the point where the curve on the screen is the lowest, this represents the x value at which cost is the least. This x value is your x answer, just plug it into the other equation to find your y and now you have your answers of how much pipe of each type is needed.
Hope this helps