How much difference does a phono preamp really make?

A good phono stage is a wonderful upgrade but with a great one the magic happens on any good recording and you do not have to spend a lot to get a great one.

Theoretical options are interesting and means a lot but practical experience is much more relevant and has more value. I used to have DL-110 about 3 years ago and paired it with Parasound JC3 Jr and Rogue Audio Ares. I didn’t really liked the cart, find it pretty average sounding with no dedicated personality. The compiration of two phono stages with DL-110 was definitely in favor of Ares. It was noticeable upgrade in SQ. The Ares has pretty firm place in my system due to its flexibility and exceptional sound signature.  

Many Phonostages share similar Topologies and when Demonstrations are carried out where a comparison is available, even if for a limititation of the time allocated for each Demonstration.

It can soon become quite evident that a selection of Phonostages sound very Similar, especially in SS to SS  and Valve/Hybrid to Valve Hybrid Comparisons.

At Bake Off's I have attended, a £300  - £600 SS DIY Build, where a large proportion of the costings is seen in the casing and casing attachments, and where a Builder will inform the audience that they had discarded a large selection of components for matching purposes, that has added to the cost,  have been witnessed to fit comfortably into the SS Category of up to £4000.

My own experience is that I have preferred the DIY Models over 
Branded Models Demonstrations costing upto £3000ish, as well as think that some are almost parity in their performances.

Branded Phonostages I have heard that are approaching the Higher £3000's can usually be seen to deliver a much improved performance and separate themselves from the Line Up.
I would think this has got quite a bit to do with the Designers 'Know How'.   
How these Perceptions would unraval, if extended listening was carried out in the Home System will also produce different assessments of the comparisons.

As said it is all about the ears as the tool to discover a preferential presentation.

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.