How about something like a quasi-log scale, e.g.,:
$19 for 6 weeks,
$39 for 6 months,
$79 for 12 months and
$99 for three years?
This is not quite like consulting/custom dev gigs, but has similar attributes. For purchases/subscriptions like yours, I've noticed that they often seem to create a tension between buying the two most likely to meet your needs - the longer higher-cost one is priced just low enough to be tempting. This probably indicates a lot of A/B testing of pricing, and decisions about whether you want to make it easier for your customers to decide, pointing customers to one plan or the other, what really drives higher purchasing levels and overall income.
Good luck!