You didn't say what CPL stands for.
So I will assume you mean Circular Polarizing Filter.
I see no wrong with using a cheap filter to learn.
But do not expect a small filter to be satisfactory on a large lens.
It will obstruct the lens and light.
You can use a large filter on a small lens by using by using step down rings.
You should not assume all Sony lenses have the same size filter threads.
For my new A5100 camera - I intend to purchase a CPL and a few ND filters. While in BestBuy today at lunch - I saw a Pocketfish CPL for $3. Yes - I know it's horrible quality. I figured it was worth $3 to learn a bit more about my camera and to learn how a very low end CPL performs.
But I noticed that it is actually tricky to rotate the CPL after threading it onto the lens. The thread size is 40.5 mm. But the outer diameter of the lens tube is just over 60 mm.
Could I get a Step-Up adaptor (40.5 mm to 62 mm) and use a filter that is basically the same outer diameter as the lens tube? Would I create issues - blocked light at wide angle - or some other issue? Seems like a CPL of this size would be a lot easier to manipulate - but I don't want to compromise something else with this plan.
I only own the kit lens now - but eventually plan to buy a few primes - probably the 50 mm Sony and perhaps a 135ish mm. I assume these lenses will also be 40.5 mm thread diameter for filters. I will not be purchasing crazy big telephotos.