I have always suggested that they add this as a micelanious quest line into the next DLC, though I didn't think of the idea of having to pay for it. I jsut thought it would be nice to get the Bandits out of there so the Jarl could rebuild it. This is a pretty good idea to get rid of all the excess gold, seen as once you own all the houses.... there really is not much need for mony at that point. What is the end game though?? Do you then govern Helgen?? Or does it become a private compound for you and your followers?? I liked the idea of getting it rebuilt to add additional side quests once residents move back in.
The bolded part is what I had in mind. It would open up a few more little side quests, nothing major obviously, but something that you could do to get the city back on its feet.
My idea would be to rebuild it slowly. Instead of just having the option to BAM RESTORE HELGEN FOR 100,000 GOLD it would be more like first you start out by building a Longhouse and barracks, and then that opens up a quest to clear the surrounding area of bandits so that you can insure that the city doesn't suffer from raids before it's even rebuilt. Then after that you rebuild the houses so you can move the citizens in, and then you can rebuild the shops and the forge and stuff like that.
The only thing that would have to be worked out is how you would go about choosing a jarl and his court, because it's not like you could be the jarl or anything. I wouldn't really know how to go about doing something like that, but once you had that in place and figured out, the citizens that moved there could start buying the property for sale and setting up shops, money that was generated to the city, which would be awesome if the game had the type of economic system that we were all lead to believe it would have.
More than likely it would have to be way more simplified but it was fun to think about nonetheless.