Without wanting to appear fascetious, depends what your costs will be. Transport of animal to abattoir; killing; transport of carcase to butcher; butchery costs - these seem to vary quite widely across the country.
Roughly, you can work on a killing out % of roughly 50% so a 500kg live beast will deliver 250kg deadweight. You will lose about 1/3 from deadweight to "in the box" so 175kg, but depends how fat it is, how good the conformation is, how long it's hung, how good the butcher is.
We sell beef boxes; so the carcase goes to the butcher to be cut and packed then we pick it up and deliver it to customers. We couldn't retail it like a shop because we don't have the facilities.