You have some garbage bulls or a death trap of a place if they only last 3 years.  As said salvage is $2500 minimum, was only a few years ago they were selling for that at sales.  Doesn't take too many $1250  calves to pay for a $5K bull let alone $1700 ones.  Even if they do only lat 3 years you should still be getting 40 calves at the least out of the bull.  That is $48K at $1200 per.  I don't have one cow that can touch that even if she stays around 15 years.

Pretty easy math

1 cow = 1 calf yr

1 bull = 10-20 calves yr at least.