Very true. It's not just breeding. Horses are expensive. Period. Between the breeding fee, board/mare care while she was at the breeders, genetic testing, vet for the ultrasound and transportation costs, breeding my mare this year has cost a pretty penny. That doesn't include just the regular farrier, feed, vitamin supplements, deworming and vaccination costs, although those expenses go up with a pregnant mare as well. You have to remember, she'll eat more and you should vaccinate at 5, 7, 9 months gestation as well as 30 days prior to her foaling date. I've got a slush fund for her should I need a vet.
One thing I have done that I've found really useful is I've found out the costs for things. I called my vet and found out how much they charge as a baseline for an exam, lameness test, ultrasound, etc. It's hard to judge if there will be any additional costs such as medication or treatment until you really get there but don't forget to allow some room in the budget for those. I have also found out what they charge for mileage should they have to come to me and their afterhours dispatch fees. Other things I've done is I've called around to different feed stores to find out what they carry and what they charge.
I also keep a running spreadsheet of what my horse has cost me. I'll be honest, I didn't start it until Fall of last year, but my plan is to keep track of what everything costs so I have a general idea, year over year, the minimum I'll need to put away for the upcoming year. Right now I'm just socking away as much as possible until I get more recorded, but I think it will definitely offer you some peace of mind if you can create a general outline of what you'll need for cash for Barbie. I even record what might seem trivial, like horse treats, but everything will add up.
Sorry for the short novel I just wrote, but I think creating a budget for your horse will be very beneficial! For me personally, it definitely alleviates some anxiety when I know exactly what I could be in for.