To begin, the Sweethome review The Best Coffee Grinder is quite helpful for background and their experimental evaluation of grinders, although of course they only tested a modest number of the available grinders.
Sweethome says what a grinder needs to make tasty coffee is to produce consistently- and uniformly-sized coffee grounds because coffee extraction is sensitive to grind size. Uneven grounds cause uneven extraction which produces overly bitter and sour tastes. Other sources agree.
My (limited) experiments back this up: The Baratza Encore grinder has 40 grind size steps, step #20 (depending on the type of beans) is my favorite for a drip coffee maker. Adjusting the setting ±1 makes the coffee noticeably sour or bitter, as verified by blind tasting. So we can expect uneven grounds to make coffee that has more bitter and sour notes.
Sweethome and other sources explain how blade grinders produce uneven grounds including fine particles. Burr grinders produce more even grounds. Particle size distribution is not easy to measure, esp. when static electricity makes particles stick together. (If you find evidence that a particular blade grinder can produce about as even results as a good burr grinder, please point it out!)
Note: Sweethome shows that the Cuisinart DBM-8 grinder has blades around a plastic burr. And indeed, they measured the DBM-8 distribution of particle sizes to be about as broad as whatever blade grinder they used for comparison.
Note: Nitpicking Sweethome's review, their charts showing the distribution of ground particle sizes would be more readable and more revealing if drawn as stack charts, one stack for each grinder to compare against the other stacks. The vertical label "Particle weight (in grams)" really means "Amount of particles (in grams)" in the given particle size range. The charts would also be more readable showing grind sizes from small to large rather than backwards. You can zoom the web page and hover your mouse over the right-side labels to make its graph line stand out.
Caveats: Are 5 measurement buckets enough to go on? Also, we're led to assume the grinder is set so that its particle-size distribution is centered in the middle of a measurement bucket. If set to center on the border between two measurement buckets, then the chart would look 1 bucket wider (out of 5) for the same actual distribution width. The main comparison chart is for a coarse grind size. Is that representative for medium and fine grind sizes?
With the Baratza Encore, 1 size step from favorite makes a tasteable difference. Baratza documents that the grinder has 40 steps from 250 to 1200 microns. Are those steps linearly spaced? Log spaced? Linear spacing would be (1200 - 250 microns) / 39 intervals = 24.4 microns/interval = 0.024mm/interval, and my favorite grind size #20 would be 0.725mm That's in the
< 0.76 mm bucket at the end of Sweethome's chart, which is fishy. Maybe the steps are not linear.
Either way, the distribution width is much larger than the step size. Since adjusting by 1 step matters, it suggests that we really want a more uniform particle size than current grinders achieve.
The AeroPress FAQ gives another answer to why it's important to use a good quality grinder:
A good grinder will grind coffee into particles of uniform size. Very fine particles block the flow of water and make it difficult to press. The same blocking occurs if your grinder is dull and produces particles of varying size because the fine particles block the spaces between the larger particles.
(The AeroPress seems to be less sensitive to grind size than drip coffee machines. Are espresso makers more sensitive to grind size than drip machines?)
While grind size is the main evaluation factor, there are secondary considerations like the ability to grind finely enough for your espresso machine, ease of use, how long it takes, and how loud it is.
Sweethome's answer to your price question
- High end: $2,700 Mahlkönig EK43 commercial-grade burr grinder.
- Recommended: $230 Baratza Virtuoso burr grinder.
- Second best: $130 Baratza Encore burr grinder.
- Budget pick: $92 Capresso Infinity burr grinder.
- Low cost fallback: $33 Hario Mini Mill Slim hand grinder.
But: These sources don't really answer the question how much to spend for your tastes? How uniform does your grind size need to be?
If you're not a coffee taste enthusiast, or if you add enough sugar to mask bitter and sour tastes, then you might be happy with an inexpensive blade grinder. Try doing a blind taste test comparison of bitterness and sourness for a blade grinder vs. a (hand or electric) burr grinder -- after tuning their grind-size settings.
For coffee taste enthusiasts, it's pretty likely that you'll want the most consistent- and uniformly-sized grounds you're willing to pay for. Of course, results that measure almost as uniform might taste the same. Again, try a blind taste test.