The 1975-S Proof Roosevelt Dime is fairly easy to obtain, as the San Francisco Mint struck more than enough examples to meet the collectors demand. Examples up to PR69 Deep Cameo condition are the norm, only in PR70 Deep Cameo are they much scarcer. But even then, examples in PR70 Deep Cameo are fairly affordable, that most serious collectors can easily obtain one.
Keep an eye out for the 1975 Proof No S mint mark Roosevelt Dime. As this variety is extremely rare. One example sold for over $300,000 in 2011.