Depends on the time frame.

This. I am more interested in the shape of the curve. For a "long time" (in Bitcoin years) the exchange rate growth was something like ~0.5% per day. Lots of booms and bust along the way but it correlates pretty well to a line with a 0.5% per day slope. Recently (last two weeks) that has jumped by 20x. 0.5% vs 10% daily that is quite a deviation. Is this the new normal (we are looking at months or years of 2% per day growth) OR are we looking a regression to the mean scenario (exchange rate eventually falls off to intersect with the 0.5% per day slope)?

I don't know but that is the question. I would buy at $10, $100, $1,000, $10,0000, $100,000 the nominal amount if pretty much a non-issue. If Bitcoin was >$100,000 for a decade and one day "Bitcoin Black Tuesday" there was a "massive" crash from ~$123,000 to $98,000 followed by a quick rebound would you buy? On the other hand if Bitcoin went up 100% per day for a week would you consider $128,000 to be a good price.

Everything that has ever been bought or sold can be a bubble and not all price increases are a bubble. One method of identifying the difference is the deviation from historical trends but one has to be aware the trend can change, another is the relative change in volume.