i argue this point with my brother in law over and over(he's that vet in Eureka Ks).
I figure you're pregging 100 cows to find 5 or 6 opens. A few years of refining reduces opens to lower numbers, but use whatever percent fits your program.
It's my observation the cull cow price swings about 15% with the cheapest time to sell cows around the holidays and the highest prices are usually in the spring and again when the institutionals get into the market the first week of August.
Many cowman use this price swing as a profit center. In any case, price appreciation will feed a cow. So what if you leave Bulls out an extra 60 days, pair out and swap your late hatchers as late pairs or at least bred up. Those will nearly sell for enough to replace themselves.