How to get MySQL to iterate a subquery in an aggregate select?
(sum(amount * (0.000+coalesce((select gas from gas_prices gp where gp.yw=yearweek(gl.stamp,0)),3.50)))) as cost
Is the answer, as provided by ypercube.

I was victim to order of operations--MySQL was taking the sum of gallons pumped, then multiplying that against the first/last result found from the gas prices sub query. As Michael quoted and noted:


  "The server is free to choose any value from each group, so unless they are the same, the values chosen are indeterminate." 


So to get what I was after, the sum of each entry multiplied against the price of gas during that week, you simply wrap that exact expression in the comfortable embrace of a sum().