All my bikes have seat bags. Remembering to move one and add or subtract specific items? That's asking for a roadside "oh s***!"
Each bike gets:
Seat bag ~ 22
Allen wrenches ~ 3 (X3)
Tire levers ~ 6
Tubes ~ 7 (x2)
Patch kit ~ 5
Sunscreen ~ 5 (but the small tube gets refilled from far more economical large ones)
So all bikes get ~ $61 of stuff
Fix gears - either a 6" crescent, 15 mm peanut butter wrench or Pedros Trixie (12, 20 or 35)
My best fix gear set up for mountains - I carry one or two cogs @ ~35 ea
Fix gears get an additional ~ $12 to ~$105
Spoke wrench or other tools specific to bike and potential issues ~ 15 to ??
I also carry: full size frame pumps on every bike. My two good fix gears sometimes get a 22" chainwhip strapped to the top tube. (Visible on my photo) I bring no boots but at least 5 bills of any denomination to serve the purpose and get me home. (If I only have a $20, I stop for espresso going out and ask for the change in ones.)