Good point on bang for the buck. But, if you are that limited by cost, you'd be running in D Stock and would have an entirely different set of priorities (except it wouls still start with driver skill and tires
).
For exhaust upgrades, I always recommend getting a full turboback that is designed from the start to work together. Don't piece together downpipe X with catback Y. To avoid semantic arguments on that point I usually say go with the uppipe first. Slightly less peak hp gains than a downpipe only, but faster spool and less lag, which is more important in autocross. You can get a full turbnoback from Nate for a lot less than $1000, including a high flow cat.