Don't try to save too much money, get a 3.0 capable Motorola or Cisco off the list above and you'll be happier. I got a good deal at Costco on a Motorola.
If buying used make sure you will be allowed to activate the modem, Cox will check the serial number and MAC before turning it on for you. Best to check the serial number and MAC with them before paying for a used one.
Giving mom my old modem was a bit of a hassle compared to installing a new one but a couple phone calls got it done.
Since you are in Vegas, if you find a price from a on-line retailer you can go to FRY's and they will price match. Outlet-Pc has DSL modems which are for Century Link (phone company). You can try going to a local COX store here in town and see what they charge. They have the Cisco modems which are good modems. Whever you go you are looking for a cable modem with the DOCSIS 3 standard which is all COX has. I am in Vegas also, their network is really good here.