As far as I was aware (when SMS had the Visitor Centre) when it was being built. The foundations were laid to support up to 51,000 supporters, so technically they could expand to that number in the future. To add on 8,000 seats, which was something being bandied around in Lowe's time, I believe they were saying it was going to be circa £3k per seat to do this, therefore a £24m outlay to bump on an additional 8,000 seats, which would have only been able to go onto the Kingsland, Northam and Chapel ends due to the hospitality in the Itchen.
Looking at it in that way, its still not a huge cost compared to Palace's and I'm sure inflation would have bumped that price to £30m or beyond, But we wouldn't fill it at the present time!