Usually, I'm all for rules governing situations, but a guide to exactly how much it costs to build a castle, done to every detail, seemed like a bit much to me.
On the other hand, since you have PCs who want to spend money, it sounds like it may be exactly what you need.
Overall, I thought the book was good, if somewhat dry, due to the long lists of materials, prices, etc. If your PCs have a lot of money, to the point where they're thinking about building castles, I would recommend this book. As a DM, I know how tempting it can be to just say that PCs can find whatever resources they need (sometimes instantly) for an arbitrary cost, and that that can come back to seriously cause you some problems. This book fixes that.
In regards to translating the book from 3E to 3.5E...it shouldn't be much work at all, really. The pricing system for things is mostly done unto itself (that is, you don't see it in any other D&D products) so it wasn't affected much by the revision. The one thing to watch out for is magic items. I can't quite recall how much magic item formulas got revamped in the revision, but this book has a lot of them. That's the only thing you'd have to watch out for.