The stallion owner has their vet out to ultrasound the mare. He finds that she is open (not pregnant). This vet also is not sure of the Uterine tone and he recommends that a uterine culture is done. He finds follicular activity that indicates that she will be ready to be bred in 3 to 5 days. Since this is the second vet to reccomend a culture, you approve the proceedure.
The Vet cultures the mare, he calls three days later with the results. The culture revealed an organisms cultured from the uterus to be Escherichia coli. He mentions that E-coli is one of the most common pathogens isolated during endometrial culturing. The bacteria is present in almost all fecal material, and it is therefore very easy for the uterus - or the swab sample - to become
contaminated. Therefore he has had a cytology smear prepared of the endometrial sample in conjunction with the culture. The cytology smear shows neutrophil presence, which indicates a uterine inflammatory response is in progress, so this indicates it is very probable that it is indeed an accurate diagnosis. The  "sensitivity" testing run which will identifed Gentamycin to be the treatment to which the pathogen is sensitive. 

He recommends an infusion of the Gentamycin, suspended in 400 milliliters of sterile saline repeated once daily for three days. You agree to the treatment and arrange for the stallion owner to meet the vet for the three days of treatment.

Your investment so far:
Item Cost
Research Costs $100.00
Stud Fee $800.00
Vet Farm Call, HC and Coggins $52.00
Fuel for trip to farm. $15.00
Ultrasound Fee $50.00
3 vet calls for infusion + Antibiotic cost $140.00
Stallion owner fee to meet vet 3 x's $60.00
Sub Total $1,217.00

As a side note, the stallion was also infected, by your mare, with the bacteria, and had to be treated with a course of antibiotics, which does not help your relationship with the stallion owner.
Your mare is now in heat, but since both she and the stallion have to be treated for the infection, the heat cycle is missed. At the end of the treatment the vet gives the OK to begin rebreeding the mare to the stallion. She comes back in heat in another 3 weeks and is bred.

You can :

A. Have the mare ultrasounded in 2 weeks to determine status.
B. Have the mare remain there and be teased back in 18 to 20 days, then palpated 3 weeks to determine if she is in foal or coming into season.