Today, 3wt Holsteins were mostly around 1.40cwt. And at 550 to 600lbs they were 1.10 to 1.15cwt.
One bunch of nice steers that weighed 360lbs brought 1.35 that's 486$ then a equally nice set at 680lbs brought 1.09cwt that's 741$ and only 255$ for putting 320lbs on a Holstein, that don't work friends.
You could keep them longer to try and make more, but I don't think they can get big enough to make it work out. Around here Holsteins just don't make sense right now, but if you have a different market for them they might be fine.