Did you ever think the words "iPhone" and "cheap" would appear in the same sentence? Well, they have. That’s because Radio Shack is selling the Virgin Mobile iPhone 4 and 4s for cheap — at least, after you consider postpaid vs. prepaid prices.

You still have to fork out the upfront cost for the Radio Shack deal, which totals $349.99 for the 8GB iPhone 4 and $449.99 for the 16GB iPhone 4s. CNET reported that buying the device directly from Virgin Mobile would run you $549.99 and $649.99, respectively.

So, the Radio Shack option seems the way to go. Then, factor in monthly plan costs. In a postpaid, two-year-contract situation, you’ll pay at least $80 per month for a contract-subsidized iPhone. But Virgin Mobile is starting its plans at $35 per month, which drop to $30 if you authorize automatic payments. On the high end, a contract iPhone will mean you pay about $1,920 over two years; at the $35 Virgin Mobile rate, that drops to $840 in the same time period.