CPL has it's own specific use.
to illustrate the extreme example of what a CPL does
On a plane pretty high in the sky. Polarising the bands of light.
Anyway, it reduces reflection (at the correct angle and polarisation), darken or lighten the sky, depending how you want to use it.
Don't use on UWA or WA, cause of banding in the sky.
Get a 77mm CPL and get stepping rings for your smaller lenses. That way you don't need to buy multiple CPL's. so just get a good one. (unless you don't intend to expand your lens collection)
CPL is usually worth 2 stops, factor that in.
Furthermore, you will need to either buy/borrow a CPL to learn how to use it effectively. And if you really use it in your photography. I almost never use CPL.
Just my 2 cents.
Nikon D90 12/24mm/ 18-105mm / 50mm f1.8
stepping rings are metal rings with an inner thread and external thread. like 67mm to 77mm.
u screw the 67 mm onto your lens and the 77mm CPL onto the 77mm thread on the external side.
Stepping rings can be step up (as above) or step down.
Yeah i recommend getting a 77mm cpl and just getting stepping rings in the future for other lenses you buy. Afterall, in the long run, professional grade lens are usually 77mm.
A stepping ring is about 10-15, depending on where u go to get em.