Zip-top bags are expensive . . . for both your wallet and the environment. There are "hacks" about how you should CLEAN and reuse them online . . . but is that more trouble than it's worth? Not really . . .
Even Ziploc says you should reuse your Ziploc bags.
A rep says that each bag should be reused "several" times, because, quote, "Ziploc brand bags are made with high-quality, recyclable materials that are strong enough to be used again." They recommend hand-washing them with soap and water . . . and then air-drying them.
But there ARE instances when you should just toss them . . . like when they've held raw meat, fish, eggs, allergy triggering foods, or when something has spoiled inside. Basically, anytime it's a food safety issue.
It also may be difficult to clean bags that have held oils and "clingy sauces." But you should make an effort to re-use any bags that held dry foods or other easy-to-clean contents.