Backpacks sprayers are great tools for pest control & weed control. Backpacks range from $50 at the home store to $250 for a top of the line Birchmeier Backpack to over $400 for an electric model that doesn't require pumping.
When buying your backpack, don't look just at price. This is especially true for professionals relying on a backpack sprayer for their livelihood. A cheap backpack that fails or for which there are no replacement parts does not save you money. Here is a photo of a cheap backpack.
Client bought a cheap backpack. It stopped spraying; they brought it to us for repairs because they couldn't find parts. We couldn't find parts either.
One look and we found a serious design problem. The spray hose connection to the pump mechanism is seriously flawed, causing the hose to kink. The kink restricts water flow, which means the tech has to pump harder and the pump will not last as long as it should.
The correct design would have included an elbow so the hose didn't kink. The elbow would have added a couple of dollars to the cost of the sprayer so it was left off by the manufacturer.
When buying a backpack, look for quality construction, good availability of replacement parts, good design for use and ease of service. For these reasons, I personally like the Birchmeier Backpack sprayers.