What is the cheapest and easiest treatment of knee joint pain? Orthotics? Brace?

Need diagnosis. There are many reasons for knee pain and therefore you need a diagnosis of the pathological structure to make a treatment recommendation. Please write back after you get checked and i can give you the answer you are looking for.
Osteobiflex. The cheapest and safest knee pain treatment is osteobiflex. It is a supplement and, as long as you are not allergic to shellfish, can help many people with knee pain. You can also try a topical anti-inflammatory cream like aspercreme. If you fail this, another cheap treatment is oral Naproxen (aleve) 1 or 2 pills once or twice a day. You can go to a good shoe store and get a gait analysis for free.
My grandfather. Always used an expression "cheap is dear". Meaning, cheaper is not always better. It might be cheaper upfront and cost you more in the long run. The best thing is to see a orthopedist get evaluated and find out the problem and what alternatives you have. The goal is to eliminate your pain and fix whatever problem you have. Not mask it, not avoid it.