Amazon offers Prime members the Pilot FriXion Point Erasable Gel Pen 3-Pack
in Black for $4
with free shipping
. (Non-members can spend $35 or more to qualify for free shipping; otherwise, it adds $6.) That's the lowest total price we could find by $4. They feature thermo-sensitive gel ink and FriXion erasable technology.
Note that it's expected to ship in three to six weeks.