I've heard Dave McGillivray advise professional RDs to charge 20% of the race's budget. For you, assuming this race has 1500 participants at $30/each ($45,000 in revenue) and nets $10,000 ($35,000 budget), that would mean quoting $7,000 (20% of $35,000).
For Dave, though, I assume that 20% figure includes some pass-through expenses (staffing) as well as some additional services (infrastructure rental) so, his net is substantially less than that 20% number. I'm assuming you're not providing staffing and infrastructure, so you should probably be closer to half that number (10% of budget, or $3500).
And then there's the fact that you're a first-time RD (right?), which probably means you shouldn't be charging top-of-market (same as Dave McGillivray!) rates. So knock off a thousand bucks and you're at $2500. To me, that seems about right.
One thing I would advise is to be careful about accepting too much of a commission-based pay scale--if you're not responsible for marketing the event (and even if you are), a lot of the race's registration growth is out of your control. But if you're responsible for the nuts and bolts of the race then you're putting in the time and energy whether the race grows or not. If you are responsible for marketing and growing the event, pay for that should be in addition to pay for doing the nuts and bolts work (permitting, managing vendors, site and course planning, race weekend logistics and race day OPS, etc).