The basic maintenance fee for a vendor starts with 20 to 24 gp per UO day. There are 12 UO days in one real day. The minimum fee per real day is therefore 240 to 288 gp.
The first 500 gp worth that you have for sale are covered by the base fee the Vendor charges.
For each additional 500 gp worth of merchandise, the vendor charges 1 gp maintenance fee per UO day.
Example: With a base fee of 20 gp and 5500 gp of merchandise for sale, the vendor charges 20 + ((5500-500)/500) gp per UO day. This works out to a total of 20 + 10 gp, or 30 gp.