You don't have to change your door speakers, the factory ones aren't bad (non-infinity ones). That'll save you about $150-$200. Here's what I've got invested in mine:
Alpine HU: $150
Infinity door speakers (F&R): $160
Infinity Basslink: $300
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Grand total: $610
So, if you just get a decent headunit for about $150-$200, and something like a Basslink for around $300 then you're right where you wanna be.
Tip: You may already know this, but if you order from Crutchfield you'll get the wiring harness, antenna adapter, dash kit, shipping, and detailed instructions on how to install everything for FREE! That's about $50 worth of free stuff.