Obviously it is virtually impossible to translate exchange rates meaningfully that far back. The value of one copper cash could vary heavily (entire books have been written on the period of silver deflation and its effect on exchange rates in the Late Qing), but was ideally around 700-900 cash per tael of silver, nominally on the lower end. As you may have noted, we are looking at a bimetallic system, which further complicates assessment of value. However it is possible to work out relative value to some extent, as from the 18th century onwards the medium of exchange for maritime trade had changed to Spanish dollars, with one tael roughly equalling 1.43 dollars (only considering weight and notwithstanding variations in silver purity). Obviously this becomes harder to translate long-term, but as a rule of thumb 500-650 cash per dollar would be a reasonable enough estimate for most of the 18th century. The table on pp.86-87 of Man-Houng Lin’s China Upside Down will be a useful resource for this as it covers 1644 through to 1911, with annual figures from 1722 onwards.