Expect to pay double the price for prime. Around here, I would expect to pay $8 to $9/lb for choice and $16 to $18 for prime rib.
Ordinarily, I'd say skip the prime and go for choice. But the decision is up to you. It will be good either way.
Earlier in the year, I happened to luck into finding prime roast {top sirloin} for about the same price as choice...$5 per pound. Oh my... was it good. It is the first time I ever bought larger, vac-sealed piece of prime. Top sirloin is usually very lean. This one had lots of marbling. I cut it up into 3 meals worth... a roast, a few steaks, and a few kabobs. My conclusion... once in a while it is worth it to splurge on something out of the ordinary.