I gave up changing chains.

I found I got about 1800 miles on a chain before it read 0.75 on the Park gauge, then I'd replace it. Keeping to that regimen, I found after 9000 miles, the cassette was too worn and I had to replace it with the new chain too.

Then one day I decided to see what would happen if I just didn't replace the damned chain.

I got about 9000 miles out of the combo of the chain and the cassette before it started being a little wonky with shifting.

So IMO the choice is, go through about 5 or 6 chains and one cassette in 9000 miles or go through ONE chain and one cassette in 9000 miles

I suppose eventually the chainring is going to wear too. I've only ridden one about 30,000 miles and never wore one out so I don't know. But I ride pretty inexpensive components and I watch for sales; I did wreck a crank once (stripped the pedal threads) but thanks to watching for sales I had an entire crankset spare that I bought on clearance at Nashbar for $8.