If any of the oxygen sensors is faulty it will trigger the engine light to come on, as the engine computer relies on the oxygen sensors to determine the correct amount of fuel that needs to go into the engine to achieve ideal combustion. If the oxygen sensor is not working, the engine has no idea if there is too little or too much fuel going into the engine which leads to higher fuel consumption and also higher emissions. If too little fuel is going in, the engine will run rough, hesitate on take off, and have less power, and if there is too much fuel going into the engine, all that extra fuel ends up in the catalytic converter causing the catalytic converter to fail and it is expensive to replace the catalytic converter. I work with Jeep and I've seen them cost $5000+ just for the part, not including labor. Don't know how much they are for Lexus.
The oxygen sensor should not be that expensive. The dealer is probably charging you way more than what it actually costs - all dealerships do that, especially for genuine parts. You can always get an aftermarket sensor or find one on eBay. Buy the sensor yourself and get the local mechanic to fit it for you - it will end up a lot cheaper.
When you get the chance, get the car scanned for codes, if there is any codes for the older oxygen sensor, save up for it over 2 or 3 months, and then replace it. But it is not good to leave it as is IF the oxygen sensor is faulty, to prevent damaging the catalytic converter. All the best!