Literal meaning: if you buy a cheap product or service, 9 times out of 10 you will be disappointed as it will not live up to your (unrealistic) expectations. Get a clue and start buying quality stuff!
Sucker's friend: "Dude, your wrist is turning green... and look at the face of the watch, it's crooked."
Sucker: "What the---, I think I got beat out of $150. This piece of shit prolly aint worth $5!"
Sucker's friend: "Well, you know what they say... you get what you pay for. Sucker!! hahahahahaha!!"