Do I need to pay deflect cost on a unit if I want to deal damage with alpha strike?

Ruling

Yes, you must pay the Deflect cost for any enemy unit with Deflect that you choose as a target for the damage dealt by Alpha Strike.

According to Rule 809.1.c, Deflect is functionally short for: "Spells and abilities an opponent controls that choose me cost an amount of Power equal to [Deflect Value] more to play as an additional cost for each time they choose me."

Reasoning

1. Targeting: Alpha Strike instructs you to "Choose a friendly unit." While the friendly unit is the primary target of the spell, the effect of the spell is to have that unit deal damage "split among enemy units."
2. Splitting Damage: According to Rule 355.14.a, if a card specifies that an amount of damage may be split among some number of units, then each unit chosen is Targeted.
3. Deflect Interaction: Because the damage from Alpha Strike is split among enemy units, you are choosing (targeting) those enemy units. If any of those enemy units have the Deflect keyword, you are choosing a permanent with Deflect using an ability (the effect of the spell) controlled by you. Therefore, you must pay the Deflect cost for each Deflect unit you choose to assign damage to.

Sequence of Play

1. Finalization: When you play Alpha Strike, you must finalize it on the chain. This includes choosing your targets.
2. Choosing Targets: You choose the friendly unit (no Deflect cost) and the enemy units that will receive the split damage.
3. Paying Costs: If any of the enemy units you chose have Deflect, you must pay the Deflect cost for each of those units as an additional cost to play the spell.
4. Placement: Once all costs (including Deflect) are paid, Alpha Strike is placed on the chain.

Summary: You are not paying Deflect for the friendly unit, but you are required to pay the Deflect cost for every enemy unit with Deflect that you select as a target for the split damage. If you cannot pay the Deflect cost for a specific unit, you cannot choose that unit as a target for the damage.

Note: I could not fully verify this ruling against the retrieved rules.