OK, first off with any calf that is that much smaller (our avg weights are usually 80 lbs or so out of heifers and it was about 20 lbs) you really have to wonder why. Genetics, disease? Could be BVD, could be some sort of genetic mutation (that's why I wonder about fawn calf). Then, it is a time management issue. IMO, there was going to be a lot of time spent messing around with that calf, I don't even think it could have reached the heifers udder to suck. Chances were pretty high that the calf was going to die anyways. And I KNOW that even if it had lived, it was never going to grow to be anywhere close to what its herdmates would be. In this case it was $$ that really played into the decision. It is a case of been there, done that.
I almost always will try to get a calf going. I can count on 1 hand the # of calves we've ever put down without working with them. We spent a week working with one earlier this spring. Was a case of hiplock where the heifer almost had the calf on her own. We are pretty lucky that it made it at all. Took 4 days for us to get it on its feet. And a couple more before we got it going on the heifer.