When teaching students to determine which item costs less, I follow a three step process to help them learn this important skill.
First, we work on choosing a smaller number. I may start off by writing two numbers on a white board and make the smaller number physically smaller (so a "2" would be written smaller than a "7") if the student is new to this concept. I try to quickly move onto the next step, with both numbers the same size, but giving the student a number line to help them if they need it. I have a variety of number lines in my classroom that I would use depending on my student's needs. Some are basic, with 0-10 in a line, others the numbers gradually get bigger, among other visual supports.