These days the mints of the world have things down to a pat science. True there are errors from time to time but in special collector coins it is very rare to find an error. That being the case most coins that are proofs or special collector mint state coins are either MS-70 or PF-70 anyway. Grade is only one factor, the other is mintage or in the case of older coins how many survived. As an example my 1916-D dime in the low grade of just good will advance in value every year, your coin may or may not depending on how many are made and just how many are slabbed. The mintage for the 2004 Silver eagle which is a bullion coin and not made to circulate had a mintage of 8,891.025 a good majority are MS-69 or 70. Over a period of time few of these will get lost or even get any wear unless mishandled. My dimes had a mintage of 264,000 and a lot have wear and some have been lost. I am a rare over the new any day person. Silver is down but even with the economy rare coins are doing real well. Hope this helps.