I need the biggest bang for my buck.
I have Leo, Mitch, Myke, and Winifred for La Junta
I have Carlos, Rebecca, ZDrone, and Robin for Sentinels.
Again, I need the best characters I can get for least amount of money.
Honestly, those listed are beginner cards. Pick one or the other. Personally, Sent's +8 bonus and good low star cards are the way to go.
Lehane and Dayton are great and affordable.John is great for the price and mono. Learn to play tobbie and he'll be your best friend. the 4 sents you have listed arn't that good. Aurelia is 7pow and cheap. Rich has good damage, especially for mono. Havok is a great play, not as good as the banned ones, but very servicable. Melvin and Colby are were a good mono sents gets expensive for beginners but worth it.
Well... the least amount of clintz you could make for a deck is 3k, any less and you've got yourself a unusable deck, now i think you can pick either go mono deck of la junta or sentinel( obviously), if you pick mono sentinel, use the 4 you have and : tobbie, john, dayton( may be a bit of farming to get him) and malia. If you pick mono la junta, use the 4 you have and : molly, and another 3 cards of choice, since besides her, none other good is really good under 600. going mono sentinel is better for you now.
Let me elaborate on my post, right now you have 8 cards, if you want a "good deck" you would not use any of those cards, for example, if you picked la junta, you would you use none of those 4 cards in a good la junta deck, so you would need to buy 8 cards anyway, similarly with the sentinel side. So, what you are essentially asking, is what is 8 cards that can be used as a deck, and is also cheap, and my answer is rescue.
If You'll be making La Junta, try Archibald, Tank, Winston, Chiro, Dean, those are under 1k and playable cards... if You can spend more, add here Burger, Gatline, and Dacote (1k) or Bryan if You can afford (3,5k)... later You should fit Emeth in, Trish is also a great card, but those two are more expensive