So DCS World is F2P on Steam...I don't know the specifics. But I do know fans of simulations may be interested...the DLC is horridly expensive and I don't know what else may be "required" to do well if anything. But here ya go...sorry if it's already been posted.
http://store.steampowered.com/app/223750/
Old post, but thought I would reply anyways.
The "DLC" are actually modules. They all run off of the same game engine and are in one install, so it is similar to DLC in that aspect. Though unlike DLC, it is not really "addon" type content. In the past you would pay $40-50 per game (Falcon 4, Jane's F-15, Jane's F/A-18 ect.) with a single aircraft. Now you simply get one world to play with various aircraft types. And it is being updated and will actually be getting a new game engine called Edge soon.
To do well you will have to spent some time doing the tutorials, maybe watching some youtube videos, and of course, practicing. Make no mistake, this is a proper game/flight sim and not a "free to play MMO". There are no unlocks, no grinds. Each aircraft has a role and a task to perform. Whichever aircraft you own you can fly. The only way to acquire aircraft is to buy them, aside from the Su-25T. Of course, there is an advanced mission editor where you can place all the units on the map. So you can play with friends online if they own different aircraft.
The only free aircraft is the Su-25T, which is a ground attack aircraft. Someone is working on a light attack/recon helicopter that he intends to release for free, so hopefully he finishes it eventually.
The next thing to consider is the level of detail for each plane. Full realism aircraft cost more and are significantly harder to learn. Every aircraft with the "Flaming Cliffs" label is simplified and lacks the highly detailed avionics.
For the best bang for your buck, you are best off buying Flaming Cliffs 3 which is not available on Steam due to legal issues with Ubisoft. You can only buy this on the Eagle Dynamics website, and it does require a legal copy of the original Lock On: Modern Air Combat (can be bought from Ubisoft.com for $10). For $50 (+ $10 if you don't own LOMAC) you get a bundle of simplified aircraft, some air to air and some air to ground: Su-25, A-10A, F-15C, Su-27, Su-33, MiG-29A/G and MIG-29S.
This is a better buy than buying them from Steam, in which the A-10A and Su-25 alone are $20 each.
The game will require at least a basic joystick, though you will want a HOTAS and likely a Track IR as well.
If modern combat proves to be too complicated, there is Rise of Flight. Similar concept, base game with two planes are free, except WWI based. My issue with RoF is that each plane is over priced and the charge extra for alternate weapon types/modifications. Good game though.
. I'll give it a try sometime in the next week. I'm trying to get Metro Last Light to work, and I'll need a break from the constant frozen black screen. 
It happens to all of us. I wish you good luck with Last Light...I'm holding off till all of you "beta" folks work out all the kinks. 
Still working like a champ. 
