Heres your options:
1) Pay like you said $10-15 a month for rently fee with tech support with your cable modem from your ISP.
2) Buy the Cable modem stright out from the Cable ISP and get tech support for your cable modem.
3) Buy your own Cable Modem NOT from your ISP any brand and not have tech support from your ISP for your cable modems.
The good... You should be able to buy almost any type of modem cable and work fine.
The bad.... If you dont get your cable modem from your ISP and you start having connection probelms down the line and they start troubleshooting your problem and see that yoour Cable modem is not one of thier own. Guess what the first thing is there going to say is the problem and they dont support it? Thats right your cable modem.
Atleast his is how its dont with @home/Insight were I live.
I'm a @ home user using a RCA cable modem and get 400K-700K depeneding on the time of day. My dad has a 3Com modem and I get better connections then him. Just hope you have a good line coming to the house with no line noise ect.. A GOOD name cable modem doenst hurt!
Good Luck!