If you want a complete set of D&D style prices then I recommend looking at Expeditious Retreat Press Silk Road. It has a comprehensive compilation of all kinds of goods and attendent price information. You can get the book here and the PDF here.
There is this which is a price list used for Harn. A pretty version is here. 1d = one silver penny = one silver piece. 4 farthings = 1 silver penny = 2.5 copper pieces.
All of this is useful because the Forgotten Realms uses the standard D&D pricing for goods and services. I personally use a Modified Harn system because there is some research behind the prices making more a more consistent system of values. Silk Road is likewise well researched although it been tweaked to be usable with d20 products especially on the more exotic/magical items.
3 D&D cp = one pint of beer = 1 farthing so the D&D price for a beer isn't that far off. Actually a little more expensive. What you really want to look at is how long it takes to make that sunrod. A 4 gp per, the price is comparable to some of the more expensive tools (a plough for example) or animals (4 sheep, or 2 pigs). But it of limited duration so their use would be limited to the more wealthy individuals of society.
One way to figure the true price a magic item is look at how long it takes to make, and how much does it cost to make. For a sunrod it makes a difference whether 20 can be produce a month or only 2. Finally magic items will be like luxury items if they can be made reliably. Think of a Damacus steel sword, silk, or a suit of articulated plate armor. All items that require a considerable amount of time and material to produce. This assume of course that magic items are crafted on a regular basis and not some special one time deal. Referee differ in how they approach. My own Majestic Wilderlands treats Magic Items as a high value luxury item.