If you're looking for an inexpensive Surefire entry, look to the G-series in the Nitrolon cases. The G2 and G2Z are both great lights, and offer a higher output lamp. Internet prices on the G2s are usually <$45-50. There's also an optional rechargable conversion kit available if you find yourself blazing through lithiums.
If you're looking for something smaller to have with you more often, look at the E2 or E2e. The standard bulb is close to the standard 2-cell lights for output, a lower output lamp is a great option, and an LED bezel is going to be released later this year for the E-series lights. The E-series lights are so handy that you'll probably find yourself carrying one quite regularly.
The E-2 in the standard anodizing (not the Type III) run a few dollars or so more than the G2. Another $5-7 puts you into the Type III anodizing. Opting for the lock-out tailcap, pyrex lens, and anti-roll bezel of the E2e will run in the vicinity of $70-75, depending on where you shop.
Those would probably be the more economical Surefire entries.
If you can snag one of the M2s that are floating around eBay fairly cheap, go for that. A 2-cell Surefire is a great platform which can be expanded, modified, added onto to give lighting options from 65-225 lumens and a rechargable option. That would be a great jumping off point for the aluminum 2-cell lights.