So... if you put off buying AppleCare for a new MacBook, MacBook Pro, Mac Mini or iMac until the basic warranty expires, the only difference in terms of service during the first year is that you only get 90 days of free phone support instead of 365.
But if you buy a Mac Pro, AppleCare includes on-location repairs... I reckon that's not in the basic warranty?
I'm not sure about the on-location repairs for a Mac Pro with just the basic Applecare plan... you might want to be sure about that.
I wouldn't put off buying Applecare on a new (or factory recertified) until the basic warranty expires, because you could miss the cutoff date, and then it won't work at all. If you're going to do it, give yourself a little lead time in case of issues with the Applecare agreement contract # if you buy from a third-party reseller.
I bought my Applecare the other day on eBay for a lot less (a lot less) than what Apple is asking. The main issue on eBay is making sure the seller is legit. Check their feedback, especially from customers who bought the same thing. That's your best clue. Also, make sure you are buying the right version of Applecare for your computer. Macbooks and Macbook Pros don't have the same Applecare contracts. Most of the time you won't get a box, but just an email with the registration contract code #, and you register your computer's serial # with the contract # on the Apple support page. If your computer is still within its first year of eligibility, and the Applecare code is legit, the registration with Apple will work, and you'll get a proof of coverage from them.
Keep in mind that you'll want to keep copies of your receipt for the purchase of Applecare (regardless of how much you paid) from who you bought it from, and if you buy on eBay, I'd suggest using Paypal and look for a Paypal guarantee for the whole payment amount, usually offered by legit sellers.
I apologize if this seems obvious, but when buying your new computer in the first place, make sure you register it with Apple, no matter who you buy it from.