The cheapest troops to train are Infantry units. If you’re looking at Infantry units, you can see that you only need food wood and gold to train them.
Why Infantry Is The Cheapest and Most Efficient Type of Troops To Focus?
- Cavalry requires
Food + 
Stone + 
Gold. - Archer requires
Wood + Stone + Gold. - Siege requires all . However, you’re going to need or training these siege units anyway for farming better.
Note: Gold is also a crucial resource in Rise of Kingdoms. Make sure that you are farming Gold because you need Gold to heal your troops. Whatever type of resources you use to train troops, they are the same resources you need to use to heal the troops, plus Gold.
Infantry Units are the cheapest troops to train!
The majority of the time in Rise of Kingdoms, you farm Food and Wood. You gather Stone and Gold as well but the fastest resources that you can farm are Food and Wood.
If you look into the Economic Technology, the main focuses of the early stage are Food & Wood technologies.
Therefore, Infantry is the cheapest unit because of the food and wood.
If you are looking for a specific unit to build as a F2P, I would suggest you to just focus on Infantry rather than focusing on the others because Stone is quite expensive in the terms of gathering. It’s much slower to gather Stone than gathering Food and Wood.
Train Infantry units more because they are cheap. If you’re going to use your speedups for events, spend them all on infantry would probably the best way.
Focus on Infantry commanders because they’re easy to obtain: Sun Tzu, Eulji, Richard, Charles Martel etc.
Sun Tzu is one of the best commanders to start with because of his AoE skills. Being an infantry commander, he can do an incredible job with 
Eulji.
Richard is an amazing commander because of his tankiness and infantry is all about tank. He also has a massive healing factor.
Charles Martel is another insane commander as well due to the shield absorption he has.
Conclusion
For free-to-play players or even for moderator spenders or anybody who wants to train their troops in the cheapest way, it’s is to train infantry units.
