How much difference does a phono preamp really make?

Straight to your question, huge difference. 

A good phono stage will make more difference, according to your tastes, then typically power cords or interconnects but whether it's worth it to you is up to you.
 I was using a Levinson 32 photos stage and liked it a lot but started using Transformers and/or Nagra VPS.  But ultimately settled on my current luxman EQ 500 because it just sounded more musical to me, Was very flexible, and dead quiet.

I've got a Technics SL-1200G with a Clearaudio Charisma MM cartridge, and it sounds amazing through my Herron Audio tube phono-stage - yes, it can make an immense difference.

Last year I built a Pearl 2, a diy phono pre by Pass Labs designer Wayne  Colburn. At first I was disappointed. Things didn’t sound much different from the built-in phono input on my Rogue Sphinx 2. But keep listening. I realized the Rogue was very good, but also what I decided was merely “pleasant.” The Pass pre looked deeper into the music. The background was blacker, with less surface noise. So there were details coming through that weren’t immediately apparent. And that’s what you’re likely to experience, I would guess. To blather on, I think you’ll find something similar with cartridges. There are very good, relatively inexpensive carts like the Ortofon 2M Red and Blue or the AT95 family. Or the Denon, which I know only by reputation. Paired with a phono stage compatible with their signal output, they do a good job at a great price. I’m currently happy with a low-output (1 mV) Grado Opus3 MI cart through my Pearl 2 pre (55 dB gain in standard build). I plan to try an MC cart, but haven’t yet because I’m inclined to agree that it’s not worth buying a cheap MC cart. I’m sure I’ll spring for a good one when I’m feeling flush.