Generally speaking, diminishing returns is really closely related to usage. If you're riding once every week or so, that's really different from if you're riding 20 miles a day. A bike that's totally unnecessary for the former could be not quite enough for the latter.

I figure for a good road bike, if you're spending more than $1500, you probably coulda cut a few corners here and there and come out about as well -- unless you're touring or racing.