Wayfinder's Guide to Eberron p. 46 has a section on "Creating Magic Items" with rules adapted from Xanathar's Guide to Everything and prices aligned with its "Buying a Magic Item" downtime activity. It suggests that House Cannith would be able to make many items at a much lower price, but common items look like they would be fairly priced at around 50 gp (probably more).
Xanathar's Guide to Everything p. 126 has the "Buying a Magic Item" downtime activity which is as far as I know the only way to buy magic items by RAW. The asking price for a Common magic item is listed as (1d6+1) x 10 gp, or half of that for a single-use item like a potion or a scroll. This isn't counting the 100 gp and one workweek of downtime spent finding a seller.
Normally there wouldn't be any guarantee that the particular item you're looking for will be for sale. To find a particular Common magic item your Charisma (Persuasion) check needs to beat a DC 10, with a +1 bonus for each additional workweek or 100 gp spent searching (up to a maximum bonus of +10). However, because Eberron is a high magic campaign XGtE suggests adding a +10 bonus to the check result which would mean that you can automatically find any given common magic item for sale.