As I found on (Takasugi Shinji's site) - contrast the difference of -tekuru and -teiku in the following: 物価が下がっていく。 物価が下がってくる。 Both mean that prices continue to fall. Both tekuru and teiku indicate continuation in this example. However teiku implies motion away from, whereas tekuru implies motion towards. You think prices are going away from you in sentence 1, such as when you don't like the trend. You think prices are approaching you in sentence 2, such as when you like it.
contrast with verb-te+iku 買って行く I'll go and buy (on the way to somewhere else)
wow! わかった。I get iｔ. Thanks
maybe this grammar entry should be more ーてくる as it is about appending -kuru to compound a verb. 買ってくる、持ってくる 等