I've been trying to get started on about $50/day. This said, I have so far been unsuccessful with PPC.
From what I've seen so far, though, it seems like money is mainly an accelerator -- a way to increase speed. By this I mean that at $50/day, it might take me several days to get enough data to be statistically significant and see if something works or not. If I were willing to spend more, I could probably get that data in a day, and thus be profitable in a much shorter period of time.
Also, you have to spend more if you're going for more lucrative offers. Stuff that has high payouts (e.g. $30+ diet rebill offers) also seems to have the keywords bid up much higher, so it's more expensive to get into. You can't find $0.10 clicks on acai and colon keywords (well, I can't, but take that with a grain of salt since as I said, my PPC efforts have so far been losers -- my successful sites are all SEO.) If you're spending $2-3 per click, then $50/day won't get you enough data to know anything, so it may be better to start with lower-payout offers.
Overall, though, it seems like the $50/day I'm spending is about the bare minimum to get data (and I'm bidding on 5- to 20-cent keywords), and may actually be a bit below the minimum.