The answers are "it depends".
Cost to open an acccount starts at something like "Free for 30 days" and then runs at minimum something like $ 49 / month and up, depending on what you want and need.
It works in conjunction with a PPC account - so you have to pay for both.
Most folks want it for auto bid control - say you want to bid on a word but only want to bid it into optimal placement on mondays, wednesdays and fridays during East Coast business hours. (Yes, you can do that).
Others just want to auto bid being # 2 at all times. (Yes, this is a bit easier and more common...)
It also enables one to turn off and turn on words and campaigns manually much easier than dealing with the Overture interface - and if you have numerous PPC accounts, you can sort of manage them all from the GoToast interface.
What you have for "keyword search" is the times that the little GoToast 'toasters' go in to your PPC accounts and check and rejigger the bids.
I call em toasters because they are like spiders only they work differently.
If you are in a very competitive area, where others are moving their bids around constantly, you'd want to get the account toasted every hour or so if you wanted to keep up with your competition.
Of course, if your competitor has their toaster set to go in every hour, you'd want to know that and set it for every hour too.
GoToast works with Overture, it does not replace it.
In fact it actually makes Overture almost manageable by adding features that the Overture folks should / could have added long ago.
It also can manage autobidding on other PPC systems - like Google AdWords and others.
GoToast, in advanced useages can track conversion, manage campaigns, calculate ROI and the like. This is what I use it for. I think it costs somewhere between $ 79 and $ 149 and up per month.
The folks at GoToast I have found to be pretty supportive. They may move a bit fast, but after a while, it is easy to keep up with them.