Having worked in the game industry as a game designer for the last 5 years one of the important things I did was provide programmers and artists with block art. This is invaluable when designing levels (block mesh), prototype art for programmers, and a loose guideline for artists.
Why spend hundreds of dollars on art when prototyping. Block art should be your only choice. Use it to quickly get a game up. Figure out all the art requirements you need, then talk to an artist for final art using the block art as reference.
The RTS Prototype Art Pack is perfect for creating a RTS game, or any game that requires buildings and vehicle units.
The pack includes:
9 ground and air units
2 turrets
8 buildings
2 resource types
25 rock pieces
4 ground tiles
Note: Since this is block art for prototyping there is only a simple png texture provided.
Web Demo: http://tinyurl.com/pygwvbc