Chain rust is just on the surface. I don't buy that rust can make a chain break unless it's been sitting in the weather for 20 years or something.

As far as I'm concerned, replace the chain and the cog if it's skipping or sucking into the gear. Other than that, just lube it and keep riding.

With my multispeed bike, I try to replace when the wear indicator says (a Park measuring tool) which for me with lots of dirt and rain means about 1800 miles. However, after a few chains it doesn't matter, the new chain skips anyway. At that point I put the old chain back on and ride it until it skips with the old chain, which is usually at about 4000 miles. I'm in that situation now; a new chain a few months ago skipped, so I got the least worn of my old chains, put it on and am still riding with it.

Also, it's a bit presumptuous to tell people what they can and can't afford. If a guy says he can't afford 8 bucks, then he can't afford it. I know people who are in that position. It's pretty rude to argue with someone who says they can't afford something.