I would say that the necessities are a helmet, a saddlebag for a spare tube, tire levers, patch kit, and a portable pump. You can go cheap on the helmet for approx 40 dollars. I would suggest getting a good pump. A Topeak road morph is highly recommended. That's another 30-35 dollars. A saddlebag can be had for about $15-$20, and the levers and patch kit a few dollars each. Maybe a pair of bike shorts and gloves as well.
Ask the shop to help you out during purchase with a free pair of their lower-priced bottle cages and bottles. A lot of them will do this for you.
Things like clipless pedals, shoes, a jersey, lights (only ride during the daytime until then), etc. are nice to have but can wait a month or two while you save up.
Last edited by Mach42; 08-16-06 at 10:05 AM.