2000 Nissan Sentra Fuel Pump vs Head gasket

Car would turn over, but not start. Mechanic estimate was $720 for new fuel pump, new ignition coil, spark plugs, oil change and the tow. Picked up car, drove about 10 miles and it won't start again. Had towed again. Now mechanic says it needs another $1100 for head gasket. I asked if that was the problem all along and he said yes, but it did still need the fuel pump as there was gas everywhere when he looked at it the first time. My question is this: Should he have known it was the head gasket all along and should he have quoted me $1800 to fix it? I never would have invested that much money in the vehicle, but I figured it was worth the $700 if it would then run. Now I already invested $700 for a car that still won't start/run. Was I taken advantage of? I am a single mother with no family in my city who could have helped me out so I'm feeling a bit taken advantage of by the mechanic. Opinions please and thank you.