I took a look at your series. I have a hard time believing all five books in the series were 120,000 words as the page count ranged from 428 to 620.
Four of the five books were set in Palatino, which even its designer, Hermann Zapf, said was never intended to be a book face. It's far too large. At 12 point, Palatino has a lowercase alphabet length of 160 points. Adobe Garamond is 135 points. Just switching from Palatino to Adobe Garamond would reduce the page count by about 18%. But one of the books was set in Adobe Garamond and was the longest at 620 pages, so it must have been much longer than any of the others.
Your lines per page range from 27 to 31, and 31 is at the low end of lines per page for an 8" book. By eliminating footers and putting the page number in the header you can get two more lines per page.
Your characters per line range from 48 to 56 (the book in Adobe Garamond is the 56). It should be about 65. Reducing the point size to get 65 characters (including spaces) on a line instead of 50 would reduce your page count by about 30%.
The average words per page for books from major publishers is 330. Yours is about 240 (Book 1). A 120,000-word novel should come in at about 375 pages, depending on how much dialog you write and how much front and back matter you have.
You can drastically reduce your page count, and printing cost, with minimal effort.
If you like 5 x 8 but want to reduce printing costs, for your next series I would recommend going to 5.25 x 8, picking a typeface other than Palatino, reducing point size, and reducing the outside margin. The paper used by both Amazon and Ingram is thicker than most papers used by major publishers for paperbacks (but not hardcovers), so don't reduce the inside margin below .75".
Adobe Garamond is not my favorite Garamond, but if you already have it you can dramatically reduce your page count over Palatino by doing nothing more than switching fonts.