Humour me here with some really rough figures re how long it would take for something like an E195 to pay for itself :
20 aircraft at 10 hrs a day utilisation = 200hrs a day x 365 = 73,000 hrs a year. 73,000 x 300 litres/hour (leaving out the fact the 190 around 5% quicker), a shade under 22 million litres of fuel a year. This is obviously not factoring in things like maintenance (an F100 would require significantly more engineers/hours per flying hour) and things like loss of customers/pax due breakdowns and rescue flights (something at least one F100 operator in Oz has a shed load of). I'm not so sure running old pieces of crap is as smart as the bean counters would have us believe.
Note : most large successful LCCs run new/or low time aircraft.