Upgrade My Turntable - How Much Do I Need to Spend?

Here is a statement I found on another website that is relevant to the mono discussion: "A true mono cartridge has only one coil and is designed to respond to lateral signals only. It does not respond to vertical signals. A stereo cartridge strapped or otherwise manipulated to become a mono cartridge still responds to vertical signals. The introduction of the vertical signal causes distortion and phase anomolies that are not fully canceled by summing the 2 signals. The anomolies are audible. That is why many consider a true mono cartridge (one coil only) to be superior to a stereo cartridge adapted to mono."

So this statement conforms to my own claim that the response in the vertical plane is cancelled.  However, it also mentions that cancellation is imperfect in that it causes "distortion" (without saying what sort of distortion) and phase cancellation (of course it does, that is how the vertical response is cancelled).  But these considerations do support the notion that using a separate "true mono" cartridge may be additionally beneficial, which in part supports Elliot's experience.  But surely the noise resulting from vertical deflection of the cantilever on a mono LP is not doubled.

@lewm 

Is it possible for you simply to admit you made a few innocent but incorrect claims earlier on in this thread? I’m sure you’re a fine person regardless. I only corrected you so as to help others who may be newbies.

Thank you all for your experience on the matter and your advice.  

On a side note, it's always interesting to see how a thread like this goes off course onto a tangent that is far from the original question:  how big of a step up do I need to make with my turntable to take full advantage of the other components in my system?

I do appreciate all of the points above and am considering these factors in my decision.

goes off course 

if a thread stays on course, we did it wrong :)

I don't think price is an indicator of quality and there certainly isn't an "audiophile range." There are many high quality turntables that can be had for much less than $4000 but the question is, would you think they're good enough? Spend as much as you and you only believe you need to in order to get the sound you want.