Difference between New and Used Ball

There are a lot of variables here so let's speak in generalities.

I play a lot of used Pro V1s simply because I am cheap. I am mainly interested in spin. In general how much better would a new Pro V1 spin compared to a used one? Assume the used one is AAA quality and let's focus on approach shots.

I am finally to where I can play a round with 1-2 balls so I want to know if buying some brand new balls will do anything for me.

I don't know that I (or anyone) can really answer your question, but here's my current thoughts on the topic.

My 'regular' course has water on every hole, most of it lateral in the areas that most courses would have rough, or maybe OB.  So lost balls are a regular occurance, even for better players which I'm not.  The pro shop sells a LOT more used balls than new, and the best and most expensive of the used balls are Pro V1s.  I've noticed that the used Pro V1s are about the same price as new NXT Tour balls (in my local shop, your milage may vary.)

The used balls look good, but I've been told by reputable sources that most of them come out of the water "somewhere."  How long they were in the water is, of course, a mystery.  However, I expect that enough osmosis (water seepage) would occure to significantly affect the ball's performance before the cover actually discolored much if any.  If cost is an issue (when isn't it?) you might consider playing the NXT's just to guard against the 'dead' ball that's been in the water too long, even if it looks good.  Otherwise, if you're concerned about the used balls, bite the bullet and play the good stuff.

Anyone with real information on exactly how waterproof modern balls are?

Used golf balls can be wildly inconsistent. It's hard enough making sure new balls are less than a few years old. A used ball might have been past its shelf life before it got lost. Once they sit out in the elements it gets worse faster. I can't tell a five year old Pro V1 from one that just came off the production line just by looking at the ball, can you? Other premium balls change the name ever couple of years so it's a bit easier to tell but still.

Buy some new ones and see. I doubt if you will see any noticable difference in spin. I play used balls most of the time and occasionally I get one that looks good but I don't think it's carrying as far as it should (probably more in my mind than for real) so I throw it in with my practice balls and get out another one.

If the cover is damaged at all or it's just looking a little worn I take it out of play.

Just buy the AAAAA off of Ebay and try to find ones that say 'no water balls'.