Eberron: Rising form the Last War introduces rules for common magic item prices on page 281, Buying a Common Magic Item:
The DM sets the price of a common magic item or determines it randomly: 2d4 x 10 gp, or half as much for a consumable item such as a potion or scroll.
This means prices would vary from 20 to 80 gp for a common magic item, with a typical price of 50 gp.
The section also discusses rules for items being available or not:
In Eberron, common magic items can be found for sale in most communities. The DM determines the stock that is available, or the DM has the shoppers make a group Intelligence (Investigation) check to find a shop or ped- dler that has the desired item in stock. The DC for this check is 10 in a city, 15 in a town, and 20 in a village. If the check fails, at least 24 hours must pass before looking for the same item in that community.