It may well have been that your great grandmother was advised against giving birth at home. Do you know how old she was and how many previous children she had before giving birth to your grandfather in 1930? You say the family were farmers so they must have lived in a rural community so maybe this could have been another factor in the birth taking place in Moat House. Just a thought regarding the cost is that Moat House offered this service for women who needed their services for a nominal fee.
Maybe it would be worth ordering a copy of your grandfather's birth certificate from Scotlands People to confirm his place of birth. The cost is £12.00 + postage and you can order it online.
Hope this helps.
Dorrie
P.S. You could contact Dumfries & Galloway Family History & Genealogy - Phone number 01387 260301 or email
www.research@dumgal.gov.uk and see if they have anything further on Moat House