Priced at $168 per device, if you find CutYourSim, a permanent factory unlock for iPhone a bit costlier. You may find Rebel's Micro Sim a bit more affordable, which cost about $65 in the pro variant or $48 for the lite version.
"The Rebel Pro Micro unlock simcard package includes 1 Rebel Pro Micro sim card 2 iPhone 4 Rebel Sim Tray."
"The former ships with a custom iPhone 4 SIM tray that keeps the electronics inside the handset while the latter option does not require a special SIM tray for your iPhone 4 and the components are located on the outside of the handset, which isn't aesthetically pleasing (see below).
Both solutions unlock iPhone 4s with basebands 1.59.00, 02.10.04, 03:10.01 and 04.10.01 and support iOS 4, 4.1, 4.2, 4.2.1, 4.3 and 4.3. Like CutYourSim, it's an untethered solution which doesn't require a jailbroken device so it should appeal to casual unlockers.
Unlike the former, Rebel's use of a custom SIM tray lets you update the SIM Software even if Apple redesigns the iPhone."
More Info: Rebel Pro Micro SimCard