If you pay the deflect cost to target a unit with deflect, and then the spell gets countered, do you get the rune back?

Ruling: No, you do not get the rune back. Countering a spell does not refund any costs paid for that spell, including the mandatory additional cost from deflect.

Sequence:

• You pay the deflect cost (mandatory additional cost) when targeting a unit with deflect
• If the spell is then countered, it does not resolve
• The costs paid (including the deflect cost) are not refunded

Nuances:

• This is similar to how you don't get energy back if a spell is countered - costs are paid regardless of whether the spell resolves
• The deflect cost is a mandatory additional cost for targeting, not a cost that depends on resolution