I don't mean to be offensive, but I'm only considering buying such an extension to save the hassle of recoding what someone else has already done.
NGUI is extremely good value in that respect, as there has clearly been a LOT of thinking and coding invested into it.
This extension to NGUI could be coded in something like a day's work (at least, that's the kind of figures you provided). Looking at the way dynamic fonts have been implemented in Unity4, it doesn't seem that one needs to go through very very difficult issues to add this to NGUI. I am a senior programmer, so it's an interesting buy ONLY ifit saves me time that I can spend on other stuff. Of course it is valuable to all the people who don't have C# expertise, so I am not saying you should give it away for free, but I would have expected a price around $20, not $200.
So I guess I'm just going to take the time to code this myself...