Ive got a 99 bmw 740iL, new cat converters, oxy sensors, brakes, water
pump, and thermostat.

The Check Coolant warning comes on when i try to drive.  I filled it
up, and in two days it said to check again-there was no coolant at all
in the tank.  So i filled it up again and the next day i saw lots of
antifreeze on my garage floor.

There are no visible leaks, ive taken the car to a trusty mechanic and
the are no obvious drips when he put it up on the lift.  Also, when i
do drive with the car it doesnt go into the "red zone"(it doesnt
overheat).  After driving it for about 5-10 min and letting it cool
down, i opened the coolant cap and there was a gurgling noise and
coolant spilled out onto the floor.
The mechanic has told me that its a gasket problem(i think head)  and
that he needs to send out engine heads to be checked and change the
gaskets.  I was told itd be about $2000(U.S.) to fix it.  Ive taken it
for a 2nd opinion to a heat/cool system specialist, they havent
finished diagnosing it, but when we discussed it they said it possibly
could be the thermostat.

Ive seen gasket parts on the internet for 50+ bucks...why am i being
told that its a $2000 fix?  Id like to know what problems have similar
symptoms and if it might be something other than what ive been told by
that one mechanic so far.  I know its a long post, so thanks in advance.