That stuff pisses me off to no end as well.
I'm in the exact same spot as you with the above. I really like Sanctum and would like to get the second Map Pack but they want to bend me over when they have already gotten more than what are currently asking for it for everything instead of giving me a break for actually supporting them.
Same thing happens time and time again for games, it really does make you angry. Or they will price the DLC at some stupid price but then bundle it with the game and it is dirt cheap (Saints Row The Third I'm looking at you specifically!)
To be fair, there are franchises that give pretty excellent loyalty discounts when buying newer installments of series you own.
So it isn't all across the board terrible.