When something goes wrong, my biggest expense is the cost of new components or the tools needed for the job. I like to do my own repairs where practical, being self sufficient in bike repairs is pretty important when it comes to bicycle being your main form of transport, plus it gives me a warm fuzzy feeling when I have a mechanical failure and the next day I'm back out on the road with zero expense. It helps to have a back up bike while you're learning how to solve your problem, as it can be a bit frustrating getting your head around some issues.
I stopped using the LBS when I had them replace a broken spoke, I was charged £25 for a replacement and wheel truing. Shortly after I lost another spoke and realised that I could replace the spoke myself and true the wheel for the cost of the component, £1.50 plus a one off payment of £3.50 for a spoke key. It pays to learn to fix your own bike.
So far, my greatest tool expenditure is the rear alignment hanger from Park Tools (£45.99 on sale), so worth it as it has saved me some serious £££ in LBS fees. In the first 3 months of owning it, I've sorted my hangers 4 times (yes, lots of crashes!). At £20 a pop at the LBS, I'm £34.01 better off. Over the course of it's life it will save many hundreds of pounds. Why people are so lazy to get their bikes repairs at the LBS I will never know!
I haven't had to venture near the hubs or bottom bracket to service them yet, but I expect those will be another set of expensive tools, but again, investment pieces.