Lambda sensors (otherwise known as oxygen sensors) are a vital component in modern electronic fuel injection systems. The purpose of the sensor is to measure the amount of oxygen in the exhaust gases spent from the combustion chamber. This is equated to the air-to-fuel ratio that entered the combustion chamber using the stoichiometric equation for the fuel being used. The lambda sensor outputs a value of 1.0 when stoichiometry has been achieved. The accuracy of the lambda sensor has a direct impact on the accuracy of the electronic fuel injection system in attaining its targeted air-to-fuel ratio. So what's the big deal? Well, there are a variety of reasons for maintaining precise control: maximizing power, maximizing fuel economy, or minimizing emissions. In short races, maximizing power will yield the best advantage. In endurance races, maximizing fuel economy becomes more important because it allows for use of smaller fuel cells or fewer fuel stops during the event. The real advantage though is if you can maximize both: in other words, increase overall efficiency. The efficiency of combustion engines is measured in BSFC, or brake specific fuel consumption. An increase in BSFC of even just 1-2% can provide a massive competitive advantage. Bottom line, a more accurate lambda sensor will result in a higher BSFC.
The dirty secret is that all Bosch lambda sensors roll down the same assembly line. Whether it is an instrumentation grade sensor (often used at official emissions testing facilities) or an aftermarket sensor, they were both made on the same line. So how can this be? Well, each sensor is tested for its accuracy at end-of-line. The most accurate of sensors get graded with a blue dot and become "instrumentation grade". These are the most expensive sensors and price can reach upwards for $400 each. The next tier of sensors are reserved for automotive OEMs and suppliers. This helps the OEM to ensure that their vehicles can meet certified emissions levels across all cars in the fleet by keeping measurement variance to a minimum. These sensors are available through official OEM channels as branded OEM sensors. The sensors available from Bosch Motorsport are also in this 2nd tier. The remaining sensors that don't meet the accuracy requirements of these first 2 tiers are then sold through various distribution channels such as the automotive aftermarket. So while these sensor can often be found in places like Amazon and Ebay for low prices, the problem is that the accuracy of these sensor can vary widely. This means that from one sensor to the next, the actual lambda (and therefore BSFC) of your engine can also vary greatly. In some cases, even the sensors that the assembly line has completely rejected can find their way into this market.
So while you can find some cheap lambda sensors out there, you are very much getting what you pay for. If you find a Bosch LSU (wide-band) sensor for under $100, it's accuracy may be pretty poor or it may even be defective.