*The views and opinions expressed on this forum are purely my own. Any product claim, statistic, quote, or other representation about a product or service should be verified with the manufacturer, provider, or party.
For extremely low volume use, which pay-per-use plan to choose isn't as important as how much money you add to the account at a time. As adidaskid said, the $100/365 days is the cheapest option long-term, but if you're uncomfortable effectively committing to a year, you can also do $25/90 days, which is only marginally less efficient.
There are taxes and fees that you pay in addition to the refill, and I don't know how that scales with the amount you pay (i.e. not sure if it's a constant percentage of what you pay, or if it's a smaller percentage if you pay more).
Unlike my previous carrier, your account balance expiration is pushed out the appropriate number of days from the day you pay, not from the current expiration date. So in order to be most efficient, you need to wait to pay until the day your balance expires. That puts a little more advantage on the $100 refill, as you only have to worry about that once a year.