The 1964 Proof Washington Quarter is very common without Cameo or Deep Cameo surfaces in most grades and easily affordable up to about PR68 condition.
Proof (non-Cameo or Deep Cameo): Most examples probably fall in this category and only in PR70 are they hard to find.
Proof Cameo: In Cameo condition it is scarce in R69 condition or higher.
Proof Deep Cameo: In Deep Cameo condition it is somewhat underrated, as fewer than 1,000 total examples in all grades combined, have been graded by PCGS with the Deep Cameo designation. Anything grading PR68 Deep Cameo or higher is considered scarce.