It's not over dramatic. I've seen 3 horse die because they were bred to "LATE" (my spell check is out, no need to be a spelling nazi. Some people are bad at math, I've always been bad at spelling.)
The sooner she gets the shot the beter. It would be best to have it done this week.
BTW you did say money was a problem when you said "Horse abortion...I can only imagine what that's going to cost."
Haven't you noticed in some of the breeding threads how often it is mentioned that the health and LIFE of your mare is put at risk when you breed? And these mares are in their prime with owners that are considering breeding their favorite mare.
If the vet is called on Monday, there shouldn't be any risk to the mare if she gets a shot to stop an unwanted pregnancy. Which is much cheaper than the prenatal care and potential foal care if she were bred and successfully carried a foal.
And by the way, my sister bred her 17 year old maiden mare last spring and she is watching her very closely with diet and exercise to improve her odds of having a healthy mare and foal in a little more than 2 months from now. At this current point in time, she is doing very well at almost 18 years old with her first and only pregnancy.