@adrian-e, I have a different approach to this.
I strongly advise against spending a lot of money on an instrument until you've been playing long enough to discover your own preferences. Some variables are more obvious than others. The VSL, or vibrating string length, makes a difference mainly in how big the spaces are between frets and the sustain the instrument has. Do you want an overlay such as ebony on the fretboard? How wide should the fretboard be? What kind of fret wire do you prefer? How far apart should the strings be? Do you want a wrist rest? a bevel or place to rest your arm while you play? Do you want a warm, mellow tone, or a brighter sound? Do you want a false bottom for more volume, or would you rather have less volume but be able to control it by muting the bottom on your legs?
You would be surprised how many small variables can make a big difference in how you (yes you, not me, because we are all different) play an instrument. It took me a few years of playing to develop most of my preferences, but I am still refining certain issues like how far apart I want the strings. Some of that can be adjusted on any instrument, but some of it can't. And you don't want to spend a lot of money on an instrument only to find after playing it a while that it's not to your liking. You would be better off playing for a while, trying out a variety of different instruments, and slowly developing a sense of what your dream dulcimer is. In a few years, if you are still playing and still enjoying the dulcimer, you'll know exactly what you want to spend your money on.
I also don't know how to quantify the differences in tone between dulcimers. I have dulcimers that cost $300 and dulcimers that cost over $1000. The more expensive dulcimers sound a lot richer and play a lot easier. But 3 times as much? I have no idea. If we are judging dulcimers by a ratio of their sound quality relative to their price, I would suggest the David Lynch beginner dulcimer to be the best dulcimer out there. It does not have the rich, warm tone of dulcimers that cost ten times the price, but the intonation is excellent, the action is good, the instrument has a nice punch to it, and the price is about a third of a Ron Gibson.
Having said all that, both Ron Gibson and Bonnie Carol dulcimers have good reputations and good resale value.