'becomes cheaper' is correct and usable in written English.
You can use it to refer to when something decreases in price, such as: "The cost of living in Europe becomes cheaper every year."
As RFID technology becomes cheaper, it is becoming increasingly prevalent in many applications, including asset tracking, passports, and mobile payment.
Marketing also becomes cheaper.
When war becomes cheaper, we'll get more war.
That is changing as DNA sequencing becomes cheaper.
As the new system reduces the air ingress the landfill gas purification process becomes cheaper.
If insurance becomes cheaper, firms will have more money to hire workers and raise wages.
But that share is expected to grow as air travel becomes cheaper and more accessible.
But with high enough usage their cost per page soon becomes cheaper.
Some companies, including petrochemicals firms, do better when their main input becomes cheaper.
Crashing oil prices could mean petrol becomes cheaper than bottled water, according to RAC.
Such thinking, though, defies the natural arc of technology, which inevitably becomes cheaper and more widespread.
