My two main bikes each get about 2500 a year and I replace the chains each spring with a new one. Usually a KMC sl10 but I'm not that picky, I'll use SRAM too. I just buy them when I see a good deal. Cassettes get changed when they skip but I ride multiple wheels on my bikes and each has it's own cassette so they a last several years.

My method on the chains works well for me because I don't ever remove and clean my chains, I just wipe and lube. But I for about $60 a year (including lube) I get good performance and have not had a chain failure.