If you are doing grass, polygons can be a possible option, 10 tris for a blade of grass and 120 for a clump isn't too expensive, it depends on how well the engine instances it though.
Around 64 for the smaller meshes and 128 for the larger ones i have improved it by turning off the cast shadows on my directional lights outside which are casting the light shafts
So, assuming that this ends up as a typical 128/256/512x6 size cubemap, resolution probably doesn't effect the quality too much, but what about high dynamic range?
Second Son is supposed to be around 120k for the main character http://www.gameranx.com/updates/id/21382/article/infamous-second-son-used-120-000-polygons-per-character/
estimated_time_spent*desired_hourly_rate*padding_for_revisions = per_asset_price. Estimate how long it will take you. Figure out what your desired hourly rate is. Multiply that by 1.5 or so to pad for the inevitable revisions.
This is your number 1 issue. Sculpting with a mouse is a nightmare. Not advised. I did it for many months back in the Zbrush 1.5 days......I'm shuddering now just thinking about it....