Understanding the arithmetic in this 1690 text
It seems that the gentlemen is speaking in the english dialect of England.  The Years purchase phrase is still used today.  

https://en.wiktionary.org/wiki/year%27s_purchase

Years Purchase: The amount yielded by the annual income of property; used in expressing the value of a thing in the number of years required for its income to yield its purchase price, in computing the amount to be paid for annuities, etc.

An example of its use is here: https://investmentproperty.co.uk/property-investment-calculators/years-purchase-single-rate/