Well, you've forgotten the 3000T and the 4000T, but lets take a step back.
Cost wise, are you saying it'd be cheaper to buy a Picasso II board for an A2000 than it would be to buy one for an A4000? No, they cost the same because most of the hardware the A2000 can use is usable in the A4000 as well.
Accelerator cards are comparably priced across the range as well, depending on the processors. The A2000, however, requires an accelerator card to be useful, while the A3000 has a 68030 built in, and the 4000 wasn't available with anything less (not counting the odd prototype '020 card.) If you want a 68030 in your A2000, you're spending $100-$200, putting your A2000 in the price range of an A3000 machine. Going up the range, 68040 cards are available for all and are similarly priced (though a 3640 is cheaper than any '040 card you'll find for the 2000,) and '060 cards are expensive for any machine (but easier to find for the 3000/4000 range.) Those Blizzard 2060's aren't growing on trees. Oh, and if you want 2MB of chip RAM in your A2000, you're spending another ~$100 for a MegAChip.
The A2000 is expandable, yes, and they require a bunch of it to be useful.
edit: I'll take a step back myself and say I'm not agreeing with those who said the A2000 was a bad idea. It was clearly superior to the A1000 in terms of expandability, but it was most definitely surpassed by later machines.
