The Squaretrade warranty (the Costco deal) will pay out the full purchase price of the set. That is, however, the total limit of what they will pay during the term of the wty. What this means is that if the set is repaired the cost of the repair is deducted from the purchase price and any future repairs or replacements are limited to the purchase price minus the cost of the repair that's already been done. The only way you'll get the full purchase price toward a new set is if your set is deemed unrepairable the first time it malfunctions. This is ok if the original set is a no-name model with no parts availability as these are pretty much always unrepairable (Vizio falls into this category most of the time). If, however, a set is repairable once or twice, as in thenightman4u's scenario, then bricks (unrepairable), all you'll get is the purchase price minus the cost of previous repairs. Given the very low comparitive cost of the Squaretrade warranty this is not necessarily a bad deal.
Most retailer's warrantys do not have this limitation, but many will not give the full purchase price for replacement. Instead, if a set must be replaced, they will pay the normal price of whatever set is most comparable to the bricked one. Some will dictate what set you'll get as a replacement, others will give you a dollar amount and let you pick your own replacement.
Also, in the case of a wty replacement, the original plan is usually cancelled and you have to buy a new plan on the new set. Sears plans do carry over to any replacement set(s).
In general, the more generous the terms of the wty the more expensive it is.