Best Buy offers Earth Defense Force 2: Invaders from Planet Space for PS Vita for $16.99. Opt for in-store pickup to avoid the $3.99 shipping charge. That's the lowest price we could find now by $3, although we saw it for $2 less in March.
Of note, My Best Buy Gamers Club Unlocked members can get it for $13.59. (A 2-year membership costs $30.)