You need an acclerator card because:
1) the only CPU that is plug-in compatible with the 68000 is the 68010 (I think...) and the speed increase is pretty much negligable
2) just changing CPUs can't increase performance that much anyways because you're still stuck with the relatively slow (and limitted size) Chip RAM on the motherboard (what is it like 3.5MB/sec?)
The accelerator cards include much-needed Fast RAM.
Yeah, it sucks that the accelerators aren't as cheap as say... old x86 chips, but I think the 030s and 020s are usually under $100. (I got up to an '030 25MHz for $35, on ebay no less). Apparently the '040 is mad fast by comparison (like twice as much at the same clock speed) so I guess those would fetch higher prices.