Let's consult ITAD on Black Flag:
Black Flag (base) - https://isthereanyde...reedivblackflag
Freedom Cry DLC - https://isthereanyde...creedfreedomcry
Black Flag Gold - https://isthereanyde...flaggoldedition
UbiSoft has a history of pricing-up their games on Steam, but selling their games cheaper that use only their own service UPlay. Only way you can get a Steam-version of AC4: BF right now is through Steam directly - so, keep that also in mind. Remember, Steam's the most popular PC gaming digital service and also that Valve takes around 30% or so of every sale on Steam - so UbiSoft often now over-charge on Steam versions of their games, these days.
Non-Steam versions (i.e. UPlay only versions) of Black Flag have hit as low as $5.22, according to ITAD.
AC4: BF has been $5 historically, but nothing on ITAD in there says which one hit $5. So, maybe it was GMG or Nuuvem, who pretty much I think is no longer tracked by ITAD?
Black Flag Gold has been $7.72 - but again, not listed where; so it could've been GMG or Nuuvem.
AC4: Freedom Cry has been $2.49 before.
I don't recommend you play many UbiSoft style games, - in a row, one after another...especially AC titles themselves, in particular, since they come out often yearly or every 2 years. Space 'em out. Otherwise, you might catch onto the formula very quickly or really run tired of the series very fast. They really don't change much in their games per iteration, TBH.
You can go to price history and show specifically what the pricing was on steam. The current price is the historical low for the Steam version. I don't know if it'll hit $5 on steam any time soon.
Also, I think Freedom Cry and one other DLC for BF have stand alone versions that can be bought.