I've worked Inova Fairfax and Alexandria CVOR some years ago. Not sure I could say anything relevant about your specialty, not that I remember much.
As to pay, have you gotten other offers for the area? That is really the only way that you will be able to tell if an offer is reasonable. Sounds low for the DC area to me, but who knows what the current conditions are.
If you are sold on this particular assignment, and have not been submitted by an agency, you can shop around other agencies. I might recommend First Assist based in the DC area (Bethesda as I recall) - they used to pay better than any other agency around for travel assignments, but they were picky about who they employed to justify premium rates to hospitals.