Did you click "extensions" on the site? Check that out and you can choose from two categories neatly listed as either web design or web development based on which kind of extension you want more information on.
Data Assist is under Web Development... just click on it... all the feature tours are there as well as much more information.
As far as WebAssist's pricing goes, you have to make the decision based on what value the tool is for you. If you're doing a lot of Data driven applications and this tool enables you to get them done in a more timely manner, than $200 is a marginal expense. If that cost seems high, than you're probably not doing enough web work to warrant it.
You could also look at the subscription program - get it and lease the tools for say, a year, and by then you'll know whether you want to continue leasing or even buy outright licenses for them.