Both Target and Radio Shack are preparing to drop their prices on iPhone 4 and iPhone 3GS units, according to a pair of new reports. Citing a pair of “tipsters,” Engadget reports that Radio Shack is dropping its prices on the 16GB and 32GB iPhone 4 to $169.99 and $269.99, respectively, while pricing on the iPhone 3GS is said to fall to $19.99; all three offers would require a two-year contract. Phone Arena is reporting that Target stores will see even more substantial markdowns on the iPhone 4, with prices for 16GB units dropping to $149.99 and the 32GB Verizon iPhone 4 sliding $249.99, while the iPhone 3GS drops to $19.99 and the AT&T 32GB iPhone 4 stays at its current pricing. The report also states that Target will be offering a trade-in promotion through which customers can trade-in an iPhone 3GS and receive $100 off the cost of a new iPhone 4.