There are 2 ways to look at this. 1) there are probably a lot of free or cheaper alternatives in the market. 2) you also get what you pay for. Vlingo is probably more polished than other alternatives and probably has better customer support.
$9.99 isn't horrible (I am not wasting my money on it though) but $20 is ridiculous. You would think they would charge like $5 and get a lot of customers instead of charging $20 and only getting a few hundred. They are out of their mind. They do know that many of the android customers are hackers (the good kind haha) and people who love to mod their phones, right?