A large conference generates and spends a lot of money. Once I got to arrange for a tiny missions conference, which meant a bunch of couples met at a hotel and paid for rooms and food. The hotels loved me after that. They phoned me every so often, even when WELS leaders refused to speak to me or answer my letters.
That missions conference alone was a pile of money, but nothing like a national conference - even for a Midwestern sect like WELS.
The congregations will probably pay for members to attend. Pastors often spend their own money as well, even though their members do not pony up for similar events at their secular jobs.
WELS is paying rent to use the facilities.
A hoard of people will buy their own food at the ELCA cafeteria and buy ELCA gifts at the ELCA campus gift shop.
This event is one more cash gift to ELCA, not unlike continued participation in Thrivent by WELS, Missouri, and the ELS. Vast sums of money given to Thrivent are used to fund ELCA, the Salvation Army (a religion), and Habitat for Humanity.