We just braught a mare in foal a couple of months ago poor girl had to travel like 700 miles to get here then she got stuck on the mainland for three weeks after she set of.
Depending on what breed of mare and how she has live (stabled ro rugged of not) depends on the accomadation you nead. If it was one of my shetlands foaling march - september then they would jsut stay in the field to foal. Molly is going to be stabled at night in february to foal as it is awful up here. If the mare have been stabbled or rugged she won't be particully hardy so I would stable her for foaling. But if the horse is native or lived out side all year round without a rug then if it is the normal foaling time (April - June) then I would just leave them out side. Also if you foal her in a stable it has to be at least 12 foot by 12 foot.
To look after them the mare just gets what ever jabs she neads (here in shetland they don't get any unless they go to the mainland as there isn't any infecus anything up here not even flu) and make sure she fed right and she should foal fine.
Other cost there will be are Microchipping the foal, head collor and stuff like that for the foal.
Apart form all that jsut keep an eye her when she is due to foal and ating different then get up ever hour during the night to check on her just in case she neads help.
Here is a shetland pony breeders website that has lots of helpful information. foaling signs
(this has A lot of info on foaling sighs) foaling diaries
(this gives you a rough idea of she should look like at different stages in gestation).
Hope this helps
Rachel (sorry about any bad spelling)