Most calves are born in spring, eat grass all summer and sold in the fall. At around 600 pounds give or take based on what they are. The grass dies, calves get sold and cows are fed for the winter to repeat again next year. Thats why you see the majority sold that way, environment makes it so.
If you want to target a different time of year, weight, or whatever you have to decide what that is and if it works locally to you. 5 to 600 tends to be the best mix of total price sold for and amount invested to reach that weight. Selling calves at 300 lbs is a waste to me, as the cow is standing around not doing anything the next 9 months.
Not quite sure what your question is? A 300lb calf @ 3.00 is $900, a $600lb @ $2 is $1200. If it just stood there and ate grass a few more months for an extra $300 then it makes sense. Avg herd has less than 40 cows,
40x900 = 36,000
40x1200 = 48,000