Promo chips come in several varieties, but the Gold Strike ones are probably the least valuable type. Someone please correct me if I'm mistaken, since I'm just assuming this based on their sister property Beau Rivage.
The chips are "use once and lose them". Let's say you bet a $10 promo chip at blackjack, and win. They'll pay you $10 in real chips, and take the promo chip away. If you had lost the hand, they would simply take the promo chip away. If you push, you get to keep the promo chip.
In addition, you are only allowed to bet these chips at "even money" bets, so you can't for example bet it straight-up on a number at roulette.
All these restrictions mean that these chips are worth less than half of their stated face value. (You may can do better on average using tricks of the trade.)