This makes sense, although I would keep it simple and just offer for example normal and extended messages (max 280 char) with extended costing for example $0.10-$0.50/each.
Condensing your thoughts to 140 characters takes effort. This would allow trading the effort to money. The per message fee would limit the usage of this feature, encouraging people to still keep things short and only use the extended feature when they really need it.
