If you use referral links, students pay the price you set for your course, unless Udemy runs promotions and you opted into them (in this case, they pay less). Basically, they pay the price that's valid at the time of purchase. You get 97% of the revenue if the purchase is made within 24 hours after clicking the referral link.
If you want to set a fixed price for your course & get the 97% revenue share no matter when the sale is made after clicking on a link, you have to use coupons.
You'll find more info at https://support.udemy.com/hc/en-us/articles/229603968-Promote-Your-Course-With-Coupons-and-Referral-...
