Generate random prices

Hi, I want to ask survey respondents if they will pay a certain price for a product, which I generated using the random number formula and stored in a variable I called price1 in the survey flow. I created another variable in survey flow called price2, which is half of price1, or e${ price1 * 0.5} If they respond that they won't buy at the first price (price1) then I ask if they will purchase at the second (price2). The survey will generate a random number for the first price but then on the next question it just says {invalid formula}. What am I doing wrong? Or is there a better way to do this? I do want to have the prices recorded as part of my data set.