I have to replace the chain every 1500 km. I replace the chain multiple times, store the old chains and then do a second run (the rationale is that putting a new chain on a used drivetrain does not work, so I need to wear them gradually). After 12000 km (4 chains in the process, 3000 km per chain) I need to replace the cassette and after two of these replacements (24000 km) even the front gears. That's 4 * 6 EUR for chains + 10 EUR for the cassette + 20/2 EUR for the front gears every couple years, or more than twice more than what you say, and that's only the drivetrain... I ride 4500 km/year (daily 2 * 7km commute + a few bigger trips during summer). According to a quick Google search, 3000km per cheap chain is reasonable.