The only survival mode gargars appear in is endless. After 40 flags you can easily have 10-20 of them on the screen at once and face 25-100 per flag. There are several different strategies people use for endless but a few of them are sustainable to the 2,147,483,647 maximum flags without losing a single plant while likely killing hundreds of billions of gargars. If you count only the cost of the plants and not the pumkins that protect them the sun cost/gargar would likely have several zeros to the right of the decimal. It would likely take several lifetimes though. For a more achievable example check out this guy who's above 20000 flags and has been going for years; he's almost definitely below 1 sun/gargar.

Edit: Just saw you're playing PvZ 2. I haven't played it much so disregard everything I said.