If you wanted to run wire over even longer distances, you can look into RS-422 converters, which is an industrial serial link, that makes use of differential signals. Just like ethernet does. What this basically means is that you get much less interference and you can run hundreds of meters of cable.
Like with the pre-molded cables: if you buy a commercial RS-232 to RS-422 converter it is bloody expensive, but if you solder your own chips it's the same price as RS-232.
The USB to Ethernet solution also works fine ofcourse, if you can spare the money.