You are understanding the rules/card correctly. What you are missing is that Skulk is a terminal action that has a very weak effect when you play it. Having too many of them in your deck will make it harder to actually play the good cards and buy better cards. As a very simple example, if you draw a hand of 5 Golds, then you can buy a Province or a Colony. If you have a hand of 5 Skulks instead, then you can't do anything at all this turn.
In other words, while the free Gold is nice, the Skulk itself acts almost similarly to how Curse would act in your deck, clocking up your ability to draw and play better cards.
As for "$10 value that you only have to pay $4 for", you are missing the fact that Skulk is NOT worth $4 as a card. It's only $4
because it comes with a free Gold. Skulk without the bottom part would be weak even if it cost
![](http://wiki.dominionstrategy.com/images/thumb/3/3d/Coin2.png/16px-Coin2.png)
or
![](http://wiki.dominionstrategy.com/images/thumb/f/f7/Coin1.png/16px-Coin1.png)
. In fact, while you can't just add up the costs of different cards like that to get a total value, if you could, then you would see that Skulk has been determined to actually be worth -
![](http://wiki.dominionstrategy.com/images/thumb/3/3d/Coin2.png/16px-Coin2.png)
, not
![](http://wiki.dominionstrategy.com/images/thumb/2/2a/Coin4.png/16px-Coin4.png)
.