If you're in the habit of only buying things for which you have the cash in your pocket, you're paying only retail value (or the sale price) for an item. A $600 computer costs $600 plus tax, and that's what you're spending on it.
If you put that same computer on a credit card, and you find you don't have quite enough to pay your credit card bill that month, you start racking up interest, which could be as much as 20 percent (or more). By the following month, that computer costs $720. Another month, and it's up to $864. And so on.
If you're not paying cash, layaway is your best bet for knowing exactly what you'll end up paying for your purchase. Simply take the $600 plus tax and add a layaway fee (usually $5 to $15).
One thing to keep in mind, though: Many layaway plans carry a cancellation fee, typically around $10 or $15 (or less for a lower-cost item). So if you decide after signing your contract that you don't really need a new computer, you could be out $15. Be sure to read the fine print.
Even if you do end up $15 poorer, your financial life is likely to look a lot brighter than if you put it on your card ...