This is an impossible question to answer as we truly have no idea. From calculations, we can assumer there were about 20,000 copies sold back in 1995 when the first issue came out. Today, we see around 1500 copies that are CGC graded. Of those 1500 copies, 409 are 9.8 and 405 are 9.6 as of late February 2016. That number is sure to change as we get closer to the AMC TV show premiere.
If 1500 copies have been CGC graded, how many other high quality copies are still floating around? The answer is not a lot. From experience with The Walking Dead, we know that most people that have high quality copies are going to get them graded to see if they can make any money selling them on eBay or another auction website.
As of February 2016, the price for a CGC 9.6 copy is around $400 and the price for a CGC 9.8 copy is around $800. Remember that supply and demand determine the price of these comic books. Even if a TV show or movie is extremely popular, if there are 150,000 copies floating around they are not going to be valued very high on the secondary market.
Preacher #1 was a low print run back in April 1995 so it has that going for it. It is also a single cover; there are no variant covers or second copies that may confuse people. This is also extremely important as it will only increase the value. Fans are not left wondering what cover they should get based on the number of copies printed.
We can safely assume there are not over 2000 high quality copies of Preacher #1 left. This means, if the show is even somewhat popular the price of the book will increase in value. You will often see eBay listings say “highest graded copy on census.” As of now, the highest graded copy is a 9.8. There are zero 9.9 or 10 copies out there. With a book that is over 20 years old, it is safe to say there will be no 9.9 copies for sale even if the show is more popular than The Walking Dead.