Awesome concept.
It is a great concept for a game. Only thing is, you can just re-use words. You can watch two different groups of letters, and just keep buying and selling the same words from them over and over. Example- i can just keep buying and selling mom, mono, nom, over and over watching just one price. You should add different modes, where each letter might have its own price, rather than groups of letters, or have the price for 3 letter words vs 4 letter words change, or be unable to reuse the same words within a certain time limit, etc. This game is a great idea but can totally be expanded to be much better. Maybe add some progressive levels or something too. You could add restrictions to what words can be used like only animals or something for a higher difficulty level.
Other nit picky items that dont matter but people will notice: the $ should come before the number, not after. umm.. i guess thats it.