Slitherine

I made note of how long it took for a city to recover after losing a PP to a French bombing attack.

Six game turns. I don't know if that is fixed or randomized.

So if you spend 60 PPs to build a bomber, and you knock out 1 PP per turn for six turns, you are costing the enemy 36 points. In 10 turns you break even, assuming you are successful every time and suffer no damage.

One of the nice things about the bombers is that their upkeep is low--1 PP per turn.

I use bombers mainly for attacking units and zeppelins (if I have any) for strategic bombing. What does a zeppelin cost? are they more/less effective at knocking down PP's? (I've never seen them reliably take out one per turn, it's more like one every three turns.)

i dont understand?
1 PP means 1 PP so in 6 turns means 6 PP
1 stregth point=10 PP its only available when you atacking convoys.
So in 6 turn you are making 6 PP damage.....that means in 60 turns you have 60 PP damage.
I think that a turn is 2 weeks....60 turns means 2.5 years...so at the beginning (low bober tech) its not ok to atack cities.
Just atack units with it...

If your bomber destroys 1 PP at a city, it remains destroyed for 6 turns. So each point destroyed costs the enemy 6 PPs.

If, and I know that bombers do miss and take losses, if you can launch 1 successful attack for each of 10 turns, you will cost the enemy 60 PPs--the equivalent to the cost of the bomber.

So, given two-week turns, it will probably take 5-6 months to break even on your bomber investment.

Remember that strategic bombing also damages enemy national morale, and you've already pointed out, they have very low upkeep costs. That means, that if you can afford them over choosing two infantry, your income will say higher, and you may ultimately be able to squeeze out those two infantry later (whereas you wouldn't be able to afford the bomber after paying upkeep for infantry).

how much dmg to morale does bombing do?

btw zeppelins are better for bombing cities, they have better strategic bombing stats + they cost less to produce (and have some nice events related ). offcourse bombers when upgraded to 3/4 tech are much more versatile and useful because they also do quite alot of dmg to units.