Packing cubes add weight. A typical set of 5 cubes empty weighs between 4 and 14 ounces (113 to 400 grams). Compression cubes weigh more than simple cubes because of the extra zipper.
Specific examples:
- Eagle Creek Pack-It starter set (3 pieces): 9.6 oz / 272 g
- eBags ultralight set (5 pieces): 8 oz / 226 g
- Eagle Creek compression set (2 pieces): 7.7 oz / 218 g
On a flight with a 15 pound carry-on limit like Ryanair, half a pound of cubes is 3 percent of the allowance. Worth knowing if weight is borderline.
The space savings usually outweigh the added weight unless you are packing very light to begin with.
See: best packing cubes with weight specs and luggage calculator for weight limits by airline.