Amazon offers the PNY USB 3.0 / microUSB Flash Drive for Android, model no. P-FDI64GOTGTO30-GE, for $17.99. (Newegg charges the same.) Plus, Prime members receive free shipping. (Non-members can spend $49 or more to qualify for free shipping; otherwise, shipping costs $8.02.) That's the lowest total price we could find by $4. It can connect right to a phone for extra storage using its own microUSB plug.
Don't mind picking it up? Best Buy charges $17.99 via in-store pickup.