Cost - a few bucks the pair, anywhere on the planet.
A soft and dark 4B Staedtler. NOT the common supermarket HARD, and useless "HB" that your Grandchilden use for school projects, that indents covers forever and the backs of stamps. But a FOUR B as shown. Soft, dark, and readily eraseable. Writes perfectly and visibly on black Hagners and black stockbook pages and glassines etc.
**ONLY** buy Staedtler for BOTH. Do NOT do the usual short sighted stamp collector mentality thing, to save a measly buck, and get some unknown and untested No Name cr*p made in China.
Buy NO other brand I urge you .... always genuine German made Staedtler Mars Plastic erasers, and 4B Staedtler German made pencils. Anything else I tried created a mess in comparison, and damaged things.
Every art supply store on the planet stocks both, (or indeed Officeworks, Office Depot, Staples etc) or on eBay - a few bucks mailed to your door.
Essential for EVERY stamp den .. also perfect for erasing the prices and no longer needed notes, that generations of cretins have written on covers and album pages etc, and the backs and selvedge of stamps.