In short, no, I don't think $100 would be enough to start with, just from buying your basic boxes alone. If you find a set, however, you usually get more for your money. I'm not familiar with the race you're playing, but check to see if they have a "Battleforce" set available for it. Most races have one of these sets, that come with a few units of troops, at least one vehicle, and maybe a commander. In the end, you save at least $30-40, depending on the race you play. Battleforce sets are traditionally $90, give or take, and will get you close to about 1000 points worth of an army. Keep in mind though that it doesn't include the Codex or the Warhammer rule book, which are easily $30 each, at least. And then to assemble and paint your mini's, that'll probably cost you about $20 to get the starting supplies you need.
I just picked up the game myself, and so far have spent about $500 total between the three Space Marine sets I have, the tools, the paint, the Codex, and a few odds and ends (ie - mini decorations).
If you're interested in trying Space Marines or Orks though, I highly recommend you pick up Assault on Black Reach. It costs $75, and gets you close to $150 worth of stuff, between the mini's, a small version of the Warhammer rule book, templates, and dice. I know said you were interested in playing Necrons though, so it's really up to you to weigh your options.
But definitely look into getting box sets if you can. Those are the best and most cost efficient ways to build a 40K army.
Good luck!