Tom is right - the only thing you will do is cause MORE damage by driving it. Worse case scenario - a good amount of water leaks into one of the cylinders and that cylinder is on a compression stroke. You try to start it - but water doesn't compress so it bends the crank, snaps a rod, etc. Not likely but remotely possible. More likely is continued overheating to the point that you crack the head or block, or warp the head beyond repair. You need to realize that since water is escaping it's likely some part of the head is not getting cooled properly.
As Tom said, the oil AND the filter MUST be changed. The filter can absorb the water in the oil and swell up, and result in low or no oil pressure. I have personally seen this happen - a few years ago on my Volvo after the radiator neck broke and it overheated (my wife was driving it at the time...)
Also, the cooling system should be flushed. Just as water gets in the oil, oil gets in the water.
It cost me about $150 in parts to replace the head gasket on the Volvo - valve cover and head gaskets, machine shop service to surface the head. I did the work myself in about 5 hours total. $1500 seems a bit high. I am guessing, but I would have expected $600 - $1000 depending on what type of shop - dealer, BMW specialist, general shop, etc.