Under current conditions, a single 8TB (actually 7.27 or some such FORMATTED) Seagate Expansion drive (usually the Seagate Archive 8TB internally) will pull in ballpark 450 BURST/month for perhaps $6 a month.

 ALL of the calculators are optimistic by quite a bit, due to how the BURST "network" is measured and an inherent flaw in that method - I generally take the figure any calculator comes up with and estimate I MIGHT get as high as 2/3ds (more commonly closer to 60%) of the amount the calculator comes up with.

 ANY size HD will work, but IMO anything under 1TB isn't worth bothering with any more.