I did a thread a while back giving advice on how people could save on their internet bills.
While I have no experience with CenturyLink, I have been using Comcast for a few decades now. Here is a link for
CenturyLink approved modems. Other might work too, they might not be listed.
I have ditched Comcast many times since 2011 when I had a second ISP, FiOS move into my area.
I switch back and forth, usually at least once a year, but sometime a few times within a 12 month period. I do this to always get the "new customer" promotion. Both Comcast and Verizon consider a new customer as someone that has not had service in over 30 days.
I am not sure this would be an option for you, but if you are truly looking to always get a good price, this might be the way to do it. The one thing I would suggest if you try this method is to have your new ISP's service up and running before you cancel your old one.
Even just trying to cancel your service to switch to another ISP could get you a good deal, my thread I posted actually stated that I do not even bother trying to negotiate a better deal do to it being a hassle in the past, but in the past year, it seems both Comcast and Verizon are both being more cooperative when it comes to trying to keep an existing customer. So, this might also work for you.
The ISPs always over charge, even worse is when a customer is renting a modem. You would probably find a much better deal online. I recently bought my parents a DOCSIS 3.0 Surfboard modem for $12 shipped to my house.
I think that many people over pay for higher speeds that they never really use nor need. A friend of mine a few years back was bragging about the high speed internet he had, I forget the speed, maybe 300Mbps up and down. He was paying three times the amount I was paying for my much slower internet.
I asked him what he did with his internet and his answer was pretty much Netflix and WoW. Just a big waste.
From talking around, I know there are a lot people that are paying way too much for speeds they would never be able to take advantage of.
All the ISPs advertise their speeds with statements like "good for 8 devices", which is very misleading.
This is one of the biggest complaint I have of Comcast versus my other option which is FiOS. Comcast's upload speed is too low. A few months back, I switched from FiOS to Comcast 60Mbps download speed for $29.99 a month. I was very annoyed with the slow and un-advertised upload speed of 5Mbps. I have a Plex server, and that is just way too slow.
I called to complain on the same day that I got my service set up, and I told them that the 5Mbps upload was too slow, and that they should advertise it. They apologized and offered their 300Mbps download speed, which included a still slow, but twice as fast 10Mbps upload speed for the same price of $30 a month.
Maybe you could try the same thing if you stay with Comcast.