It's perhaps a little bit more difficult ... do you really want to add coins every 10 secs or add for example 1 coin for 1 house 10 seconds after "this" house have been built and then 10 seconds after etc ... ?
In this last case seems the better way is to use instance variables for objet "house" and "flat" and use this instance variables as timers for each element built ...
I saw a recent post about someone requesting help to make ennemy fire X seconds after "spawn" and not all at the same time and seems someone give him a good commented answer ... 10 seconds i'm searching ;-p
Found ! answer comes from "R0J0hound"
Consider rhouse and rflat as sprite objects ... (in fact here "rhouse=realhouse") you have many instances of rhouse built and many instances of rflat too ... each objet have a instance variable called "timer" set to 10 by default (10 seconds) .... also a global variable "coins" to count "coins" ;-p
on every tick
--- For each rhouse
-------- rhouse : timer > 0
----------- rhouse : Subtract dt from timer
-------- rhouse : timer <= 0
------------ add 1 to coins
-------------rhouse : Set timer to 10
--- For each rflat
-------- rflat : timer > 0
----------- rflat : Subtract dt from timer
-------- rflat : timer <= 0
------------ add 10 to coin
-------------rflat : Set timer to 10
and so on for each type of built element ...
C2 is very powerful ! ... you can also create a instance variable to manage delay after which a element begin to get old and make earn less coin or will be destroyed !
But perhaps the more interesting feature is that rhouse timer and rflat timer can to set to different values ...
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But if you only wanted a simple 10 seconds timer for all elements managed with the same is simply
--- Every 10 seconds
------- system : add houses + (flats*10) to coins
(where houses flats and coins are global variables)
simply add "+ (numberofnewconstructtype * newconstructtypegivehowmanycoineach10)" for each type of construct you will have to this formula
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Don't be afraid by number of events spend to release the heart of your game ... because this part will make your game interesting or not .... your don't have to repeat "on every tick" or "on every 10 seconds" each time spending 1 event for nothing, try grouping events using the same condition.
Good sim city like project ! ;-p
ENjoy !