I've been both student and instructor for local autocross schools, and they frequently represent a great bang for the buck; we had about 8 runs for $60 IIRC.
If your region does them, test and tune events are great. Since it's not a competitive event and runs are practically unlimited, getting one of the "fast guys" to ride along shouldn't be hard. Local test and tunes are $25, so if you're looking for lots of autocross seat time, they're tops on my list.
I had a terrific experience with the Evolution school. It's typically a little pricier than a local school, but working with national-championship talent is pretty nice. The exercises they use have been developed over time and really emphasize the lessons. Unless you're a rank novice, I feel the lessons gained from the Evolution school are worth every penny. (See the "Higher Learning" article from the November issue for all the details on our trip to the school.)
Telemetry is great—as long as you're willing to spend the time to interpret the data afterward. It can also be hard to get enough data from a single autocross event (say 4 competition runs) to really analyze. To really see where your driving technique could improve, you'd want to compare your best run against a faster driver's best run, so the logistics can be a pain while collecting all the data.
Don't discount a good book, either. Clicky here...