There's no single "average price" for a DS Lite case because the market spans everything from $2 neoprene sleeves to $50+ rugged hard shells with screen protectors and game storage. Here's how to navigate the range and find what you need at a fair price.
Don't overpay for "collector" packaging if you just want protection. A $12 hard shell with 6 game slots and a screen protector does the job perfectly. Compare 5+ listings before buying.