Great questions about bow rehair and repair. To answer your first question, I believe it is all up to the player whether they decide to replace a bow by getting it rehaired, or decide to just get a new one. I would say though that under $100 in value, it doesn’t make much sense to rehair a bow because typically the repair cost to rehair is above $40 (might as well get a new one). That is totally up to the player though, as you may be really attached to your bow.
The way I handle it is that if a bow is under $150, I will toss if it needs to be rehaired. This takes a while though, as hair only dwindles when it is played often (unless you toss your bow into a celing fan–which I have done before).
Regarding when you should consider getting your bow rehaired. I suggest considering a rehair once the hair of the bow is below 3/4 the amount of hair from when you got it new. To make sure you understand how much that is, take a new bow (maybe while you are at a string shop), and see the comparison between your bow and one with a full rack of hair. This comparison looking at it from the bottom of the frog angle is what I’m talking about regarding your bow having 3/4 hair.
The harder you play on your bow the more you are will find that it will lose hair (totally normal). I would say if you practice 3-4 hours a week that you will need to get your bow rehaired after 1-2 years. This would mean that you would lose about 1-2 hairs a week, as there are many hairs you would have to lose to get down to 3/4 amount of hair.
Hope this helps!