It depends what style you're going for, to be honest. You could probably get away with 20-30 tiles per area (~5 flat terrain + 4 roads/paths (the various tiles can be rotated) + 10 obstructions (buildings, trees etc) with leeway for some extra "detail" tiles) if you create strong generic tiles and are planning on an overhead view.
The time required therein would be a week or three to get one area polished, and the money would likely be ~$10 a tile -> at least $800 for your still environment assets being quite modest.
If you're keen on an overhead view you can cut asset costs by rotating actor (mech, infantry, w/e) sprites in the direction they're facing, and this also makes explosions easier with particles (though if you're on a tight budget with individual sprites you'll want fewer and larger explosion particles rather than lots of small ones and procedural detail).
The hangar view is a good idea, but it depends on your target resolution (and whether you want x2 pixels inside that resolution) as to whether pixel art is entirely viable. Regardless of the resolution needed, if you just want fairly large stills that's no problem, animation is where you'll start paying through the nose at large sizes, though there might be some artists around who're keen to animate at large sizes, however in english we call such people "masochists"
I'm fairly interested, fwiw.
Any figures relating to money are estimates and relate to me, others are likely to have potentially higher or lower rates, with varying quality/experience.
