Dis donc's a discourse marker, and as such not really translatable by a single word.
It's used as way to signal to one's interlocutor that the sentence it's attached to has left a strong impression on the speaker or that they're impressed (in a good or a bad way) by it. That's why you see the translations you cited.
T'es costaud, toi, dis donc ("You're pretty strong, aren't you" with the nuance they don't quite look it -it's often used for kids)
Y'en a de la foule par ici, dis donc ("Wow, There sure are a lot of people around here")
Ça a dû coûter bonbon, dites donc ("Man, it must have costed a leg and half)
Faut pas vous gêner, dites donc ! ("Don't you have any shame about what you just did?" An euphemistic expression literally meaning "It's not necessary to shame yourself")
With this meaning, I have a very strong tendency to put dis donc at the end of the sentence.
Another usage is to catch the attention of someone as you initiate a conversation, particularly if it's to scold them or speak to them aggressively (The related "dis / dites" discourse marker merely signal that what will follow is a question). Then it's often at the start of sentence, quite logically since it prefigures its tone:
Dites donc, vous ne feriez se taire vos enfants ? (Hey you, could you keep your kids quiet ?)
Dites donc, là, qu'est-ce que vous faites ici ? (You there, what are you doing here?)
Discourse markers often show important regional variation, so it's possible other speakers use it differently.