Very true, and also the cost to make 1 blade is the sum of a fixed cost (inspection, paperwork, tooling, etc) and a variable cost (materials and labor) so that a 5 bladed head with blades must cost at least 20% more than a 4 blades head.
The vibration goes down by a ratio of the 2/3 power of the number of blades, so that the difference between 1 and 2 blades is enormous, 2 to 3 is big, 3 to 4 is less so, and so on. By the time you get to 7 vs 8 the number is too small to count.