It is a fact, assuming that you're not wrong and chicken sandwiches are actually pricier.
A fact is something that is absolutely, no doubt about it true. You cannot argue a fact, a fact is not subjective, and if you disagree with a fact then you are wrong.
Using your example, if you read the prices off of the menu of the restaurant and it says that the chicken sandwich has a higher price tag that the double cheeseburger, then it is a fact that chicken sandwiches cost more. If someone told you that they cost less, despite what it straight up says on the menu, then they would be wrong.
Now, an example of an opinion would be "chicken sandwiches are better than double cheeseburgers." Opinions are matters of personal belief, not what is the black and white truth. Someone can disagree with an opinion and this does not make them wrong. If someone tells you that double cheeseburgers are better, you cannot prove them wrong like you can with the price. Instead, you would have to argue your point to try and change their mind.
Of course, if there is no menu to point to to back up your statement with, if you are not stating something that is true, then it's not a fact, it's not an opinion, it's an unbacked claim, aka you're wrong.