I'm not a fan of renegotiating after the 1st BCD. Certainly, an SB may force the discussion, but as Ali2 notes, it means the issue of control is not settled. I prefer to negotiate upfront with the assumption that we will have an ongoing arrangement, knowing that either of us can bail at any time (it is "no-strings" after all).
So some observations/thoughts about this post:
1. "Exclusive" is fantasy. We know it, she knows it. But she may want to offer it (falsely) as a negotiating tactic. She incorrectly assumes we will care about other dicks using her parts - and we don't; as long as she showers and gets tested, please. So acknowledge when she offers, but assume (and don't care) that it's a lie.
2. I did post about the power of good DATY. But that still lays (pun intended) on the foundation of cash paid for intimate time.
3. Control is yours to keep or give away. You always have the power to say no. And you are correct to do so in this case, but...
4. There are a few approaches you can take IF you want to renegotiate your arrangement.
A. Propose more frequent dates as a means to get more funds.
B. Since she is focused on the cash, offer to drop the date extras like expensive dinners and gifts/shopping
C. Meet her half-way, then drop the date extras anyway.
NOTE: Underlying all of these strategies you must base your proposals on YOUR BUDGET, not her "worth." Never tell her she is not "worth" $600. Tell her your budget makes it difficult to commit to anything over $400 for an ongoing arrangement. She will never agree she is worth less. Let her find a way to "fit" into your budget, and therefore you own the "control" valve for the arrangement.
Finally, arrangements always end. I suggest you not focus too hard (pun intended - again!) on keeping this one, as there will be new SB's in your area soon. I'd rather have fond memories of a few great, hot, sloppy, wet, earth-shaking BCDs with one SB, than several memories of uncomfortable, awkward, is-THIS-really-worth-an-extra-$200-when-there's-a-sweet-18-23-year-old-hottie-I-could-be-banging-for-less moments.
Life is good
The Cat