The FPS maps would be at least 4x4 square kilometers and come in several different terrain types (hill, plains, river, lake, etc). I'm curious how much expertise is required to code and if its expensive and very time consuming to have done.

Are you using an engine?  Engines like Unity or Unreal have lots of procedural landscape generation tools on their market-places.  Some of the nicest ones are under $100US..  Then it's just a matter of inputting the settings and textures/etc..  This kind of task can be greatly simplified by using a modern gaming engine like Unity or Unreal. 

If you are talking about a full "own code" approach using a custom built terrain engine, then it's likely to very expensive and/or time-consuming.

I would suggest checking out this site for some resources.

http://pcg.wikidot.com/

I personally will generate height maps for terrain, but it comes with limitations for things like caves. There are tricks and ways around this, but to be honest I usually just cut a hole through the terrain and insert a cave mesh, some people use layers as well.

You can also look into these topics: Perlin Noise, Perlin Worms. Also research Voxels, Marching Cubes and Dual Contouring. You have many options, it just depends on how your engine is setup. Some games will use 'hybrid engines' where they use polygons for the characters, and objects, but voxels for the terrain.

You'll want to be comfortable with a shader language as well.

4 hours ago, Septopus said:

Are you using an engine?  Engines like Unity or Unreal have lots of procedural landscape generation tools on their market-places.  Some of the nicest ones are under $100US..  Then it's just a matter of inputting the settings and textures/etc..  This kind of task can be greatly simplified by using a modern gaming engine like Unity or Unreal. 

If you are talking about a full "own code" approach using a custom built terrain engine, then it's likely to very expensive and/or time-consuming.

I'm sorry I should have mentioned. Yes, the plan would be to use Unity. I'll look into the offerings in the marketplace. Thanks.

I'd say it depends a lot on your background and interests.  Procedural content is a special interest off mine so.  If it really interests you that you'll probably put a lot of time and thought into it, but enjoy it and find the challenge enjoyable.  If that you're just wanting the results, such as for unlimited replay on large open worlds you may just want to use some of those tools from the marketplace.

I posted a large answer about that some time ago to a similar question

https://www.gamedev.net/forums/topic/692945-creating-a-3d-world-for-an-rpg/