Stamp dazed: To answer your first part: Depends on the issuing country, the issued pattern rarity factor, and the common variety of stamp the Perfin is setting upon. On US stamps, Perfins are rated between a "F" grade up to an "A" grade pattern (for a total of 6 different grades). If an E or higher rating item is on a VERY common stamp, the value if the common stamp is exceeded. If the pattern is on a higher valued stamp, the price of the Perfin will lower the stamp price and often exceed the actual rating value of the Perfin.
If the Perfin is on a complete envelope cover, the price of the Perfin is most always placed in a "more valuable" category.
The price of a Perfin is also changed by the country selling the item. Example, a US perfin will always go for less money in the US than say anywhere in the EU region. It is all supply and demand. Generally, the pricing of any countries stamp goes for less in the US than in the country of foreign origin. This also includes foreign material selling within the US for some reason.
Pricing is also most always higher than the true value of an item is on eBay
The second part: There are catalogs of most all Countries issuing Perfins. Most are expensive items and are issued by organizations around the world. If you plan on getting into the collecting of Perfins seriously, I would consider joining the group in your country (or others) that specialize in Perfins as the catalog prices are usually reduced to members. The US has a great group, the "Perfins Club" and the link was in earlier reply to you.
Hope this helps.