Basics

10-50 gallons per 1000 sf roof need to be diverted: [1]
For 1250 sf Seed Eco-Home roof - 12-60 gal needed
Examining prices below, it's best to use 6" pipe - more storage, lower cost.
If store 30 gallons - 30/600 = 5% of rainfall in 1" of rain
1" of rain is 800 gallons or $8 at a rural water rate of 1 cent per gallon [2]. That is $320 value of water per year just from rooftop catchment.

Sourcing