A single yellow highlighter can currently be purchased for only 38 cents, and I don't know how volatile the highlighter market is, but you will probably pay more or less for a highlighter when you actually purchase it. Now for 38 cents you aren't going to get anything fancy, but you are going to get a tube of plastic enclosing a bunch of florescent yellow pigment that can be easily dispensed when you want to.
A single orange highlighter can currently be purchased for only 43 cents. Why is orange exactly 5 cents more? Nobody knows. Maybe making the yellow highlighter 5 cents cheaper is a nod to the vast Yellow #5 conspiracy. It clearly isn't a matter of supply and demand that makes the yellow highlighter cheaper, because everybody always wants yellow highlighters. I can only assume that it is a matter of the economics of scale. Integra makes so many more yellow highlighters that they are able to produce them slightly cheaper than the orange ones, and when you consider that their markers are almost identical, you have to really be manufacturing a lot more yellow markers to make up that difference.