I have a PC that's interested in crafting his own foci, but we can't figure out how to do it. We've got the basics down, but can't determine how many reagents he will need to use.

Street Magic's very clear about how many you need for fetishes, talismans, and ritual materials, but says absolutely nothing about how many you need for foci of any variety.

So question 1: How many refined/radical reagents do you need to make a focus? (Note, I'm not asking how many you need to get the bonus on the Enchanting roll)

Question 2: What's the difference between reagents of the same variety? More specifically, why would anyone use venom, valued at 600 per unit, over blood, valued at 50 per unit?

As far as I can tell, you only add reagents when:

1) The GM tells you to because it fits the item (Optional Rule: Exotic Reagent Requisits, page 83, Street Magic).
2) To add dice to your Enchanting Test, though the reagents must be Radical (page 84, Street Magic, Crafting Foci, third paragraph).

Radical being the naturally occurring type, rare pieces of metal, mud or other natural material found in distant pristine wilderness harvested in the dead of night, under a full moon, on the equinox. (Yeah i just having fun there, but you get the idea). Alternatively Exotic material, Dragonheart string, Unicorn mane, Phoenix feathers, Vampire's teeth, or The hand of famous serial killer, things with magical significance . Generally the monetary value thing is less important to the value of the story. (IE when your players character determins its time to make the ultimate sword of ultimate doom, the components should be appropriate to the item.) As with most magic stuff you generally want to avoid them coming across as being bought off the shelf at the stuffershak for 10k So when he/she goes to purchased Talismanai then be sure to describe the gooy bits.