Suppose you have two numbers. The difference of the two numbers is 12. The product of the two numbers is 17. Multiply the larger of the two numbers by 100, add 50 times the smaller number to that, round the total to the nearest whole number, and that's my price."

you have two numbers;
a and b

the difference is 12
a-b=12

the product is 17
ab=17

price = round(100a+50b)

solve the first equation for a
substitute into the second equation
solve this for b
substitute into the first equation and solve for a
substitute a and b into the price equation
done!