I just saw this question on my blog post about inks:
Are the Rubine Red, Pantone Yellow, Process Blue, and Opaque White the only colors you use to mix with? I also have a PMS Guide and want to avoid spending $100 ordering 1lb cans of ink in all the colors so I'm trying to figure out which basic colors I can get by using to mix the PMS colors.
First, there's a great discussion on ink on Briarpress that you can look at that gives you tips on ink. I found it particularly helpful. You can order some lithographic ink to save some money, which tends to come in smaller cans and therefore cheaper. However, since it's oil based, it will dry in can, which, along with ease of cleaning, was a huge concern for me when I was deciding what to do in the beginning.
As you know from the post about ink, I started off with Rubine Red, Pantone Yellow, Process Blue, and Opaque White rubber based ink. I got black as samples from a bunch of places, and I also purchased metallic gold (from Accent) and silver (from Van Son) oil based ink because I have a thing for metallics. I chose these particular colors after thinking about what colors I would initially be using and going through the PMS color guide and seeing which red, yellow, and blue matched my needs best. I still use these colors extensively, but I've found that I have needed other colors as well.
Luckily, I have a friend who is also starting her own letterpress studio and her initial needs were Red Pepper (PMS 199), Pantone Yellow, Pantone Green, and Reflex Blue. She also decided to get a mixed Chocolate Brown (a must in this design time). Then we met and traded inks. I figured out later that I really didn't need Red Pepper from her since it's based on Rubine Red, but I've used that as a basis for Joie Studio's pink (added white) and red (added black) business card colors since it's easier than starting from scratch. I also mixed the Pantone Green with the metallic silver for the blue-green coasters I did recently. I also just stole a little bit of Warm Red and Pantone Purple from her.
Next on my list to acquire will probably be Warm Red and PMS 8560, but I'm waiting for that design project to start before buying.
My advice to you is to look through the book and decide what colors you absolutely need, then buy more colors as you go along to spread out the cost of ink. At the end of the day, $20-$25 every now and then isn't so bad. My initial purchase of those 4 pantone colors plus the metallics probably set me back well over $100, so it's not cheap, but I figure that the rubber-based inks will last me for a long time, as opposed to the 1/4lb cans of oil based inks that may dry up on me before I get to use a significant amount. In letterpress, a little ink goes a long way.