Parking at the 2025 convention host hotel is rather expensive at $22 per day. If you Google “overnight parking in downtown St. Paul”, you will find 6 or 7 parking garages within 4 blocks of the hotel, all of which are less expensive. You can make an on-line reservation at most of them.
The convention recognizes that none of the options are cheap, which is because the convention is happening in the downtown area of a large city.