As far as I know, the only aftermarket bags that are widely used are Arnott, and you should have no fears at all about using them. I would NOT use any other aftermarket brand. And these are quite a bit less costly than BMW OE bags. Since a pair of the front Arnott bags alone is $720 +shipping, it sounds like a corner is being cut somewhere. You have to figure a charge of a minimum of about two hours labor for changing the bags and calibrating the system. So to me, I would expect at least $950-$1,050 would be more than fair assuming you're using the Arnott bags (not struts...just bags.) Even before the bags start to leak, you will see cracks start to appear in the folds of the bags. With your car's age and mileage, it's time. You should budget to do the back ones soon as well. Those are considerably easier to install, and the bags are much cheaper.

Keep in mind this is not a repair to put off a single day once you have a leak like this. You will quickly ruin the compressor by forcing it to work overtime. And that is $$$$$$. Even in the scenario where the leak is really coming from a fitting somewhere, it's time to do the bags anyway. Sometimes you can see the leak if you spray it with soapy water or Windex, but sometimes if it's leaking in the fold of the airbag, you really can't tell. Once it's off in your hands, it's easy to tell if the bag is cracked.

__________________
2014 BMW 328i Xdrive
2011 BMW 335i M-Sport
2008 BMW X5 4.8i Sport
2000 BMW 528i 5sp