Need recommendation to eliminate the $10 monthly rental fee that I pay for my Xfinity 8305c wi-fi cable modem router.?

Solution
Depends on your needs where you want to buy it from but just about any dosis 3.0 modem with wifi will work. just depends on if you want a basic one or ones with lots of features like time restrictions for devices and limit speed to some computers.

then follow this to make sure it will work with your package https://customer.xfinity.com/help-and-support/internet/list-of-approved-cable-modems/

before buying
Depends on your needs where you want to buy it from but just about any dosis 3.0 modem with wifi will work. just depends on if you want a basic one or ones with lots of features like time restrictions for devices and limit speed to some computers.

then follow this to make sure it will work with your package https://customer.xfinity.com/help-and-support/internet/list-of-approved-cable-modems/

before buying
 
Solution
Keep in mind if you replace their modem and router, if there are any issues with your service you may need to replace and troubleshoot the equipment yourself. If you own the modem, they will not help you with it past checking their side of the connection. Generally the cost is offset since you normally won't have issues all the time, and even a year of that service fee will buy you a new router or modem.
 
Funny this should come up in a thread, I literally just replaced my Comcast modem the other day. I purchased one of these:

https://www.amazon.com/ARRIS-SURFboard-SB6141-DOCSIS-Cable/dp/B00AJHDZSI/ref=sr_1_2?s=electronics&ie=UTF8&qid=1485798762&sr=1-2&keywords=surfboard+modem

I got a higher end model, but I believe this one will work too. You can shop around to see what you want in terms of speed, and wireless capabilities, but make sure you check whatever model you pick against the list of Comcast approved equipment.

I will tell you from my experience, you are making an excellent decision. We were paying the $10 a month to rent the modem, and then additional for 100 megabits down. I did a speed test before changing anything, and the old modem was giving us about 30-40 megabits down. We dropped the rental modem, replaced it with a Surfboard modem, and upped our speed to 200 megabits down. I installed the new modem, returned the old one, and did a speed test on our network. We are now getting 233ish megabits down.

And as far as service, there are actually some third party vendors that Comcast will service themselves, including some of the Surfboard modems. You'll have to check which ones they still service, but I know there are some.