I have a PC hooked up to my TV for web based content, with a wifi keyboard/trackpad. It's a bit tedious to use at times but it's worth it. If I didn't have a PC I would consider casting, but I wouldn't be interested in a browser on the Roku even if it were free. Unfortunately more cable channels are requiring authentication before you can stream from their site i.e. we can't watch new episodes on MTV.com anymore because my cheap cable package doesn't have MTV. So we buy them on Amazon. (still cheaper than upgrading cable)
Web browsing with a roku-style remote seems like a fail. Years ago I used the web browser on the Wii a couple of times. It was also a fail. Sooooooo many links to navigate past if using the D pad (most analogous to how a roku would navigate). Scrolling up and down on a page was a PITA. The Wii was lousy at rendering HTML, but even if it worked well I would not have bothered browsing on the Wii because the interface sucked.
So, no I wouldn't pay for a browser on the Roku. I wouldn't bother installing a free
browser on my Roku unless there were a compelling reason to do so. And there isn't.
It's a curiosity, something to look at and say "gee whiz", and forget about later. Not something to spend money on or even use for free. I'd bet there are a lot of channels on the Roku like that...