Chris Walton already answered how to use a single circular polarizer for your two lenses of different sizes. But I want to address the implicit question "why are there such different prices in circular polarizers?"
One of the reasons for the large difference in price between polarizer filters is quality. More expensive filters probably have better coatings to minimize reflections (which is important for polarizers, because they have 2 planes of parallel glass, which can create their own internal reflections which show up in your images as flaring or ghosting).
Another possible difference in quality that could account for price differences is possible color cast. Some manufacturer's filters tend to add a slight amount of magenta, blue, green, or amber color cast (depending on manufacturer and brand).
When you are considering specific brands or models of polarizers, search the internet for reviews and example pictures produced with them. You'll find plenty of both. That will help you make the value judgement between low-price and higher-quality CP filters.
See also:
- How do I choose a polarizer?How do I choose a polarizer?
- In which situations should I use my circular polarizing filter?In which situations should I use my circular polarizing filter?
- How does one use circular polarizers?How does one use circular polarizers?
- Circular polarizer works great at shorter focal lengths, but significantly degrades the image at longer lengths. Why?Circular polarizer works great at shorter focal lengths, but significantly degrades the image at longer lengths. Why? (multiple answers to this question suggest the answer might be caused by cheap CPs)
- Do I need a slim or normal polarized filter for a wide angle lens?Do I need a slim or normal polarized filter for a wide angle lens?
You mention that,
There are some "popular" websites that are selling these for really cheap along with a UV filter,
Regarding UV filters, there are lots of opinions both for and against their use. Rather than run down those opinions, or insert my own opinion, here are some Photo.SE questions about them:
- What effect does a UV filter provide?What effect does a UV filter provide?
- Is a UV filter better for lens protection than a protector filter?Is a UV filter better for lens protection than a protector filter?
- Should I put UV filter to protect the lens even if I put a lens hood?Should I put UV filter to protect the lens even if I put a lens hood?
- Are there any downsides to using a good-quality UV filter?Are there any downsides to using a good-quality UV filter?
- What are the consequences of stacking a circular polarizer on top of a UV filter?What are the consequences of stacking a circular polarizer on top of a UV filter?
