A good reference for plate blocks is Brookman.. excerpts from catalog
http://www.brookmanstamps.com/pricelists.htmlThey have prices for sheets and plate blocks. Now, their pricing is high (meaning you should find it cheaper elsewhere) but is useful for relative prices.
The number is usually 4 unless some special reason (like a second plate number). Most collect by the format defined Scott (4,6,etc).
Example excerpt from Brookman's web site for educational purposes only
