I just got 2 rokus planning to cut on cable bill but here is my feedback on all this
I really do not think Roku can substitute cable. For one noting is live. You do may for each subscription. So lets break it up
Netflix[movie only]- 7.99$
Amazon Prime [movie only] not a great selection - $79/year so Roughly 7$
Hulu Plus [tv shows] - not all of them are on it, sometimes they are late and some channels are not on it like Travel and Food and HGTV - $7.99
So you are already paying $23+ taxes each month
Add to it the cost of Internet. You really need a high speed internet for this, maybe cable or satellite. It really does not work optimum on DSl. Additionally you definitely need a wireless router to use it. SO I am taking part of the internet [I pay 42$ each month for comcast 15mbps internet] so I am taking $10 as my cost for internet on Roku
I am at roughly $35 for roku. Now keep in mind if you have kids there are not many kid channels even paid ones. They disney one is like videos and music so no Dora and no Phineas and Ferb and no HGTV, Travel or Food. Also no live sports or NFL tickets and such.
So what is an ideal scenario?
You can get Roku and then lower your cable level of subscription. I pay 90$ on directv for their choice HD plus package + 3 dvrs + 1 standard receiver + cinemax. Call your cable tell them you are thinking of cancelling and then see if you qulaify for discounts. Tell them openly to justify you keeping them. They will get your bill closer to $100. That plus $35 each month for roku or less if you subscribe to fewer stuff will save you money.