If you are looking for the cheapest, most affordable ATP-CTP course, you will do yourself a favor by checking out this list of all FAA-approved ATP-CTP providers. Go through the list and search each school individually to see what their prices are. Some will indicate the price of their ATP-CTP course, while others will force you to send them a message.
As of October 2020, prices range from $2,900 to $5000+ for the courses. We would have listed each school and their prices, but prices are subject to change. Here are the 3 cheapest as of this writing:
ATP school, at $3800, is an honorable mention because 4 of the 7 days of training can be completed online, saving you quite a bit of money in hotels.
GlobalOneTraining has an ATP-CTP course for $2900 if you complete training by the end of 2020, but we are not sure what the non-discounted price is.