You paid $40 for Arq, though. Assuming you intend to keep it updated, that's $30/year¹, which buys you 120GB-month per year on Tarsnap. It also works everywhere where there's an Unix shell (including Windows with Cygwin), not just Mac OS X.
¹ (assuming 16 months between major versions, as in 3 → 4)
Sorry, I wasn't clear. I didn't meant you could get 120GB of storage for a year, I meant you could each year buy 120 units of "GB-month" (like kWh but for storage).
You could distribute that equally throughout the year, using 10GB-month per month, yes. Or you could use 1GB-month in the first month, 2GB-month in the second, etc, as your storage needs grew.
¹ (assuming 16 months between major versions, as in 3 → 4)