That seems like a lot of repairs for about 3,750 miles in American terms if what you said of 1,000km per month. You should not have to replace anything except tires. So maybe the miles, or kilometers you wrote I misunderstood? Sounds like you're rough on your bike. There is no reason to be breaking teeth on cassettes, replacing all those derailleurs etc. The only repair you should have had if you started commuting on a used bike would have been tires, one chain, one cassette and one chain ring. There's no reason to replace tubes like you do if you patch the flats which would have saved you at least $50 in tubes.

I have a bike with over 150,000 miles on it and the only stuff I had to replace was usual wear items like tires and tube, brake blocks, chains and freewheels, one set of chain rings, rims last about 35,000 to 40,000 miles, and cables...that's it.