St Giles Cathedral says it has 314 seats as a music venue - but there is plenty of space not occupied by seats. St George and St Andrews on George Street has a capacity of 250. So 300 seems like it would fill a large church.

Say a Church of Scotland minister costs £40k a year, then each of the 300 people in the congregation need to be chipping in about £150 a year just for staffing one minister.

I suppose the sustainability depends on how much of an endowment the church has. If I'm reading the latest accounts correctly they have about £50m in income from investments.