This seems to have been an intentional breeding?? If so, I would hope you would have put a bit more thought and research into it than to think a human pregnancy test would function correctly. Human based tests are designed to detect a hormone associated with human pregnancy - they will not work correctly on non-humans. There are tests designed for the hormonal changes associated with horse pregnancies -ie WeeFoal. As for blood testing and scans, the prices vary from region to region and vet to vet, the best approach is to contact your vet and inquire directly with them. Having a good relationship with your vet is going to be key in a healthy pregnancy and delivery for your mare.