I think it should be more about getting sponsors to provide prizes for the tournaments, same we had in last Summer Cup. In my opinion iff we now implemented a buy-in in order to be able to play, we would probably see some decrease in the number of teams attending the tournaments. And I don't think that's a good thing at this point, when the community is still seeing fast growth.
As @jtorro1233 said, there is a big number of players who do not want to pay anymore than the price of the game itself. And consequently, in my opinion there are still big skill caps inside Lite, Pro and Elite, which could to some extent decrease the motivation of "worse" teams to attend tournaments if they needed to pay for it.
But altogether very nice idea! And I too think that there should be some movement towards developing the leagues to have some prize system but it's hard to tell which is the best one in this case. Maybe we can take some examples from other esports games, where these kind of prize systems have already been implemented.