Hi there fat_freddys_cat_77 and welcome to the site.
The value of books is really quite complicated and I doubt if anyone here can help without seeing the books in question and the signatures.
You have to bear in mind that Terry is a hugely popular writer and there are therefore more copies of his books produced in a print run than a lot of other authors.
A date like "1989" is not necessarily an indication of a first edition as several editions can be produced in a year. And then you have to take condition into account. A buyer would want the cleanest copy possible -preferably one that had never been read before with a perfect dust cover.
That's really what it boils down to when it comes to value - how much someone else is prepared to pay for it.
Terry is very generous with his signatures (and suffers for it with repetitive strain injury) and it has jokingly been said that a first edition hardback WITHOUT Terry's signature would be worth more than one with it, because every known copy has been signed. Whether that is true or not is any one's guess.
Buyers are going to be looking for pristine first edition hardbacks with a signature relevant to the publish date (Terry's signature has changed over the years) that is not a personal dedication. Someone called Bert isn't going to want a book signed "Best wishes Mary, Terry Pratchett" - unless they are buying the book for someone named Mary. But "Best Wishes, Terry Pratchett" would be more desirable.
You might want to check out the prices that the books go for on Abe Books
Hope that helps.
Wyrd bið ful āræd.
"Coffee. The finest organic suspension ever devised. I beat the Borg with it."
- Capt. Kathryn Janeway