Providing it's not the Cubs in town you will have no problem securing a ticket out on the street from both professional ticket sellers and just regular folks with an extra ticket. This is especially true if you are just seeking one ticket. It is not legal in Missouri for anyone to sell a ticket up to the face value. But in my experience you should get a ticket for $5 and no more than $10.
Best corners are 8th and Spruce just across from the 3rd base side gates and then also 7th and Cerre just by Paddy O's. But in reality there are people everywhere with tickets.
If it's for a weeknight game, especially non-rival, non-division I would certainly pay no more than $5 for ANY ticket. It won't matter you can sit practically anywhere you please. If they ask for more just walk on. If it's a weekend, division rival than maybe $10 at most.
Don't bother with Craigslist, Stubhub, etc...
Though keep in mind there are some Stubhub coupons out there that may allow you to get tickets relatively priced.