Currently, there is an SDP that allows admins to set minimum donation amounts - DON_MINIMUM_MANUAL_DONATION:. It may just be for Donations Classic, I'm not sure.

If you add an "Optional" amount to donation forms you can also set the minimum amount for a transaction. But that is the minimum amount for both a one-time transaction and a sustaining gift. A $5 minimum may be acceptable for a one-time gift, but not a monthly recurring gift for 12 months (0.42/month).

It would be nice to be able to set a minimum amount for a one-time transaction and a minimum amount for sustaining transactions that would go in to effect when the user selected a sustaining option.

Nature Conservancy seems to have this active, but it appears to be some custom development as opposed to out of the box functionality

https://support.nature.org/site/Donation2?10420.donation=form1&df_id=10420&intc=nature.tnav.button