Foreword: I'n not an expert about MMOanything or games in general, but I have a decent grasp on security and I wrote more than a single client/server app. Also being an avid gamer kind of helps.
Your conclusion about the inability to protect your game is essentially correct, at least if you expect it to be 100% cheat free: whatever logic you delegate to the client has a chance to be bent by a very determined player (and if money is involved, odds are you'll find some...), whether because you are trusting the client (eugh...) or due some glitch in the validation code you might use if you don't.
You could in principle make a game completely cheat free by delegating almost everything on the server side, and turn the app into a plain graphical interface that just shows the battle outcomes, and accepts inputs (with heavy validation too...).
Point is, while this approach is quite easy for gambling and very slow paced games (board games, for instance), it quickly becomes unpractical for all others, especially when visual delay is important (think about bullets, for instance), and unless you greatly simplify the RTS mechanics of your game, odds are this approach will not be suitable for you.
So this is what you can do to make your game reasonably cheat free: if your players compete against each other in a way that it's disadvantageous for all players to cheat at once in a same game (for instance because they gain nothing by favouring a player over another, or because the pot comes equally from all player funds) you can make each client validate the data coming from the others.
I'll make a simple example: suppose you delegate all bullets physics to the clients (a reasonable assumption), you can make sure that each client keeps tabs on the others or a subset of the other clients (as a full check would scale like O(N^2), even though reducing the checks would greatly reduce the security of the system as a whole).
As soon as a client detects someone firing at a rate not allowed by it's level or by the game rules in general, or some bullet reaching destination faster than expected, it will rise a red flag by the server and mark the whole game as invalid.
At this point the game can be cancelled (no money paid), and the offending player can be marked as a potential cheater (if it's the first offence) or banned, if he has been repeatedly marked (and some external check verify his fault, otherwise malicious players could use your own system to ban first class players... See how tricky this thing is? :) ).
Mind that this is a very basic example, with a very basic response from the system (which in turn might allow cheating by abusing the rules. A player might use this to cancel a limited number of unfavorables games to get a better shot, or just for griefing).
The topic is very complex, and not being an expert I'll stop here, I still hope to have gave you an idea about what expects you if go ahead
Hope this helps
