Let's see... $1.33b to put up a summoning circle... $266m refunded for the factory you tear down... Net cost to build is $1.064b... It's tricky since there's no more conventional plunder cap like there used to be, but let's assume the plunder + ally plunder from Circle to be ballpark $500k (it's totally less than that, but...)... The 5% bonus on $500k is a whopping $25k extra per attack! So $1.064b divided by $25k means you only have to win 42,560 battles before that Circle's 5% bonus has paid for itself.
OH YEAH!!!! I'ma put up a bunch right away!! Lol.
Even if you don't look at it that way, that the bonus has to pay for itself for it to be worth tearing down a factory to build a circle, if you count the entire plunder of the circle (which makes no sense, since you were already getting the same plunder except for that 5% bonus from a factory), if you count the entire $525k plunder, that's still 2,026 battles required before the circle is paid for by it's own plunder.
In short, it makes absolutely no sense to tear down factories in favor of circles for the extra 5% plunder. BUT, if you're putting up a brand new building, then it's worth thinking about.
I was excited to switch my 4 factories for 4 circles until I did a little math.