I went through Chrome Flesh and Data Trails. If a page had a rule on it, the page number is included below. (So I'm counting as "rules pages" a lot of what people might call "fluff pages".)
Chrome Flesh: Pages 25, 50-65, 71-93, 107-124, 145-167, 176, 179-193, 222-238
= 114 "rules pages" (48% of a 240-page book)
Data Trails: Pages 43-49, 51-53, 55-66, 76-77, 86-88, 90-91, 93, 95-96, 98, 100, 102-105, 110-120, 145-165, 178-182
= 75 "rules pages" (41% of a 184-page book)
Both of these Core Handbooks are over 50% fluff (Data Trails is nearly 60% fluff), even counting entire pages as "rules" that really only have rules in a tiny sidebar or for a single statblock.
For comparison, Man & Machine (SR3): Pages 13-50, 54-60, 62-79, 90-100, 105-124, 126-136, 138-140, 142-158 (Plus an index, which I'm not counting!)
= 125 "rules pages" (more than 3/4 of a 160-page book)
And Augmentation (SR4): Pages 20-23, 31-49, 60-71, 86-94, 105-117, 120-135, 151-176
= 99 "rules pages" (56% of a 176-page book)