IIRC, Jim stated that Harry has about 2mil of diamonds and I doubt he would want to ask Mab or Molly for more a loan. Those interest rates have to bite big time.
Two million is one tenth of the $20 million that Nicodemus said they could easily get. The diamonds were split 5 ways, and then Harry split his 1/5th with Murphy. That's an annoying amount because it should be way higher. Like 5 to 20 times higher. I think we've all seen/been party to the conversations about how much the diamonds should be worth. Here's Priscilla's math on how much the diamonds would be worth.
https://www.reddit.com/r/dresdenfiles/comments/273l6q/skin_game_spoilers_money/chxf3vf/. I tend to agree with her. Now, you would still have a limited, but large, budget because you probably can't just sell all the diamonds at once, especially when you consider that there are four other sellers out there unloading these diamonds. Five if Murphy's share doesn't end up back in Harry's hands.
Jim also said the reason for the diamonds was to move Harry past having money problems and into the mo' problems mo' money brings.
But Harry is a knight now and marriage to a rich heiress is the traditional way for impoverished nobility to improve their financial position. 
In had a friend who described owning a house a bit like owning a whore.. 
Was it a rent house? Because I get that analogy.
And he's not rolling in income either.
This. He doesn't have a job anymore.
Maybe he'll get lucky and the events of BG will have made property values in Chicago plummet.
They would make the average value plummet, but the value of an intact and fortified residence would probably skyrocket. I'm not sure if they take actual value into account for tax roll appraisals.
But renovating a castle has to be insanely expensive. How many people do you know who have experience building, maintaining, or repairing a castle? I'd expect that alone to make it expensive. The materials aren't going to be cheap either. Granted, Harry only has to fix the roof/ceiling in a single room, but maintenance is also going to be expensive. I've seen estimates ranging from 1 to 5 percent of the market price of the castle.
Harry might be able charge a fortune to people who wanted to rent a room from him. The castle's going to be really safe. It's coming and going that would put residents in danger.
This all would be a lot easier to figure out if we knew how big the castle is.
