Does Flurry of Blades target units? Do you pay the cost?

No, Flurry of Blades does not target units, and you do not pay recycle costs for Deflect when playing it.

According to RiftJudge FAQ #1583:

• Targeting: Flurry of Blades does not target units. It programmatically selects all units at battlefields rather than having the controller choose specific targets.
• Deflect: Because it does not target, it does not trigger the additional cost imposed by the Deflect keyword (Rule 735.1.c).

Additionally, Rule 352.10.d states that objects are not targets if they are "programmatically selected based on its characteristics rather than chosen by the spell or ability’s controller." Since Flurry of Blades affects "all units at battlefields," it falls under this exception.