First, how old is this mare? Is she broke to ride? Can you use her for pleasure once you have bred her and raised the foal? Or do you plan to sell her on? Why are the owners getting rid of her? Don't trust anyone, everyone is nice when selling something to you, get a written stud contract with a live birth clause. Better yet, If you decide to do this, have the owners breed the mare and buy her with a certificate from the vet that she is guaranteed in foal. Also with a live birth guarantee if possible.
Second, be sure about the breed registration - some registries are now demanding DNA tests from bloodwork. The Tennessee Walking Horse Association went from hair samples to blood work and foals out of registered TW horses without a blood sample for their DNA were no longer acceptable to be registered. Make sure the mare and the stud have all their paperwork on file with the breed registry. Just because they are registered does not mean that their foals will be eligible for registration. Check this out.
Third, unless this mare has a fantastic pedigree and show/performance records and wins, and is bred to a superlative stud with the same, the foal will bring hardly anything. It takes 2-3 years before you can actually break and ride the horse for any length of time. Some earlier and some later - depending on the breed some legs and backs take longer to mature.
Fourth, are you able and do you have the finances to keep the foal until it is breakable? Do you have the money to break the horse? Do you have the time necessary to work with the foal?
14 years ago we bred our TW mare. We had a terrific stud, and a live birth contract. The foal was born dead but the vet verified that it had never taken a breath so we were able to have the mare rebred to the same stud. The second foal was alive but very weak. The mare didn't have much milk so we had to supplement. My husband imprinted the foal, and bottle fed until the mare's milk came in.
BUT before we could breed either time, we had to have a vet do a clean check for uterine infections, $$$, then we took the mare to the stable for a live cover (better in my opinion than messing with AI for a maiden mare). Stud fee and boarding fee $$$$ - and remember we had to do it twice. The second cover was free but not the boarding. $$$ Then of course, there are the vet checks to verify pregnancy, more vet checks when she gets near time to foal, $$$ extra feed $$$, and for us more vet bills $$$ because the mare didn't have a lot of milk and the foal was weak.
The mare was a beautiful mare with exceptional bloodlines. She hadn't been worked for a couple years when we bought her and her previous owner had tried to do jumping, etc. instead of proper TWH stuff. We put her with a trainer (turned out to be one of the best flat shod trainers in California) Sue worked with her and then asked if she could show her a bit. We agreed and she won all her classes (flat shod) every time she was entered. The stud was a national winner. The foal turned into a gorgeous little filly and after Sue trained her she was a standout in the show ring as well. We were not interested in showing and asked Sue to sell her. We priced her at $3,000. No takers although everyone that saw her loved her. We finally gave her to DDIL1 who lives in Nipomo and rides with a group that all ride gaited horses.
We figure that we had $10,000 into her from the time we bred to the time she was broke and showing.
If you want to have fun with breeding and foaling out a mare, enjoy. If you have pulled calves and can identify when the mare might have trouble, go ahead. I delivered a stud colt from my neighbor's registered Standardbred mare. Hard work but I had delivered about 100 other difficult births in sheep and goats. The foal was just a lot bigger.
f you don't have the money, time, or experience, don't bother. Too many people decide to breed their mares and then don't bother having the colts or fillies broken when they reach 2 years old because they don't have the money.