You already know to heat your lemons/limes/oranges before you juice them because warm fruit yields more juice. What you want to do is get them HOT, not warm. I used 5 small organic lemons and got about 2 teaspoons short of the full cup that was needed because I got the cut lemons red hot in the microwave then juiced them with a reamer.
This cost me $5.54 with tax and it made a quart- $2.09 for store-brand corn syrup, $2.99 for a bag of 5 Nature's Promise organic lemons, and 19 cents for bottled water from a gallon container.
Other than the tartness, I have no complaints and because of the extreme tartness I give this 4.5 stars and not 5.