You can buy an excellent DOCSIS 3 modem for $85-$100. It's more expensive up front. But the longer you keep the modem, the more you save. I know people who have had the same modem for 6 years or more.
If you decide to buy, check out your service provider's support pages for recommended DOCSIS 3 modems. If you stick with top brands such as Motorola, Zoom, Linksys and D-Link, you'll get reliability and a good warranty. You should also be able to use it with another provider if you move.
I called up to have my speeds increased to Ultra in Northeastern MA. They said no problem, but my SB5100 wasn’t going to work and I needed a new modem. They sent me a [Motorola] SB6120 DOCSIS 3 modem via UPS the next day as part of a self-install kit… With the new D3 modem, I am seeing 30+ on powerboost down and a steady 5 up. The new modem definitely makes a difference.
Call your provider and see if it will upgrade you to a DOCSIS 3 modem for free. It might do so to keep you a happy customer. Some people have even gotten the monthly rental fee waived.