A B-Movie isn't really a measure of budget, but rather is a genre all of its own.
For example, by your criteria The Blair Witch Project would be considered a B-Movie due to its tiny budget and low production values, but this is not the case. At the other end of the spectrum, we could consider Pacific Rim to be one of the biggest budgeted B-movies around, so it is worth examining what constitutes a B-movie.
This wiki link contains a lot of interesting information, namely that the B-movie has evolved from being the second (or 'bottom') movie in a double-bill into a class of its own - glorified and emulated by seasoned professionals in the industry.
Another great example to consider is Mars Attacks which is a supremely well-crafted, big budget movie with its roots firmly in B-movie territory.
At the end of the day, the whole B-movie genre is rather ambiguous, often (as you suggest) at the mercy of critics who like to pigeon-hole films. For the record, most of the movies coming out of the Asylum or SyFy stables are considered B-movies, and it is true that their budgets rarely exceed $1.5M and the production quality is sometimes horrendous.
It seems the whole genre hangs on the whim of critical reception and filmmaker intentions, and probably less to do with investment.