As far as odds go, I can't help you. But there is a good method for gaining coins (I'm the same way, I'm too cheap to buy the coins)
-Save at a slot machine
-Start playing. Remember how many coins you started with!
-Play for a little while. Try tapping in time to the music, and always bet 3.
-If you get even one coin above your original total, stop and save.
-Repeat.
-Tip: You can find some randome coins by searching the ground and also talking to people.
-Tip two: If you get three sevens on a machine, I have found that it's more likely that it will happen again, so STICK WITH THIS MACHINE.
-One more thing: The prices of pokemon are different beetween the games, so if you have both, buy it on the cheaper game and trade (For instance, in LG, Porygon is 6,500 coin, while in FR it is 9,999 coin)
Hope this helps!
And if you don't feel like you have time for all this, get the Amulet coin, and just buy whatever you need.
pikapras