Does it have to be purchased directly from GW, or from w/e retailer you want? Also, if you don't spend some dollars in a month, does it carry over to the next month?
If the answers to these questions is "Whatever" and "Yes", then I would look online to find the now out of print "Dark Eldar Battleforce", which comes with:
10 Kabalite Warriors
You can find these online for between 90-100 dollars with shipping. So, I'd double your alowance for month 1, and get one of these. Then buy an archon. I'd save the other 30 dollars you have left to get another battleforce next month. Then, I'd continue to save money to buy either battleforces or shardstorms. They're the best bang for your buck, since you actually get the models cheaper than retail in these bundles.