jmc

Subtracting the two equations gives: $$\begin{array}{rl}(729x+731y)-(725x+727y)& =1508-1500\\ \Rightarrow 4x+4y& =8\\ \Rightarrow x+y& =2.\end{array}$$Multiplying this equation by 725 and subtracting this equation from $725x+727y=1500$ gives $$\begin{array}{rl}(725x+727y)-725(x+y)& =1500-725(x+y)⟹\\ 2y& =50.\end{array}$$So we can write $x-y$ as $(x+y)-2y$, which equals $2-50=\boxed{-48}$.