Seconding everything 32 said.
When you buy pre-made assets, you're one of dozens, hundreds, maybe thousands of people who pay that $100. They become worthwhile for the artist only when enough people buy them. When you commission custom artwork, you have to make it wortwhile for the artist with just this one purchase. It will cost much more, but you can lower that price by making the job easier for your artist.
In your job post, make sure you're clear about what the game should look like. Tile size, projection (RPG view? Isometric? Side view? etc), the sizes of key game objects, whether you want a blocky tiled look or a more organic one, etc. Mention what sort of game it is, as different styles of gameplay have different conventions and requirements for the artwork.
It'll also help if you explain what sort of feeling you're going for with this area (dark and gloomy, bright and happy, desolate, lively, realistic, alien, etc). Provide examples of games you like the look of, so that the artist has an idea of what you're going for.
The clearer you are about what you want, the quicker and more easily an artist can make it for you.
Later, once you've found an artist, you'll probably want them to spend their paid time drawing rather than asking you questions. To this end, you should try to provide as complete and detailed an asset list as you can once they've accepted the job (or in the original job posting, if it's a very small list).
For example, if you're making a top-down RPG-ey game and need tiles for a typical grass-dirt-cliffs-trees environment, don't just say "environment tiles" or even "forest, grass, cliff, dirt tiles", except perhaps as a summary - explain what sort of transition tiles you need if any, how many of each tile type you need (variants), provide a list of objects you need drawn and how big (in tiles or pixels) they should be.