Hi!
Your situation definitely requires very close follow up. If you were my patient, the first thing I would do is to repeat the ultrasound in my office. A good quality transvaginal scan should be able to detect a pregnancy when the hcg has increased over 1000. If the ultrasound showed the pregnancy, then the answer would be clear. If not, and if you were low risk, and if you had no symptoms or discomfort, I would be willing to wait 48 hours to see what another hcg level would be. By this point the answer should be clear.
If the pregnancy is normal, the hcg should be well over 3000 and definitely visible to ultrasound. If the hcg doesn't increase enough, that is a sign that the pregnancy isn't viable anyway.
One more test to consider is a progesterone level, that can sometimes help determine viability of the pregnancy.
Laparoscopy is also an option, but I would encourage waiting.
Methotrexate is much easier cheaper, and less likely to cause more damage to the fallopian tubes.
Hope this helps!
Dr B
Get a second opinion first, that way you'll feel better about the decision and you'll have several options or opinions instead of just one. Hate for something bad to happen but also hate to eliminate a possible baby. Good luck to you.