Although Feep only paid 5k for Sequence, consider two things:
1. He programmed the game. Even fresh out of college programmers generally make at least 50 or 60k a year, though some might work for a discount to be working on a game. How long do you think development will take? If it's a 2 month development cycle, then 10k (half of your 20k budget) sounds reasonable and if you find someone willing to work for cheap it could cost less or give you a slightly longer dev cycle.
2. Sequence didn't require a large amount of art assets. I haven't played the game, but Feep mentioned that the artist was essentially just paid to make one-screen backgrounds for fourteen levels and then a few other assorted things. You say your project is a simple 3d unity project? So you need a modeler and texture artist (could be the same person), not just a 2d artist, and depending on the type of game, a lot more art assets than Sequence did. Games that have entire levels consist of one static background (tower defense, music/rhythm, puzzle) are certainly doable for less than 10k, but games in which you're roaming around a 3d world generally require tons of things modeled.
Based on those two things, 20k seems reasonable if the type of game you're designing falls under certain constraints, but could be undershooting it otherwise. My advice would be to, as you're designing the game over the next x months, try to make a rough asset list, even if just for one level, to help give you an idea how much work an artist will have to do to make your game.
