How much is a repair for 335I coolant hose leak?

So you have a rear heater? If not, it is not a coolant leak. You can by pass the leak by cutting the hoses before the leak and splicing the two hoses to each other.

To fix an engine coolant leak buy an anti-leak product and add it to the coolant. Next solder or use a high-temperature epoxy to fix the leak, and replace any broken hoses.------------------------------------------------------------------------
Renault CAN Clip Diagnostic Interface

Hi Robert, No it is not normal for the coolant level to drop at any time. The BMW has a system which must be bled to free it of all air. The bleed screw is situated on the radiator and has a screw driver slot. Open the bleed screw and replace the coolant with the recommended coolant and then finish topping up with water until all bubbles have cleared from the system. Observe the level and if it drops again have a pressure test carried out on the system as if the level drops again, there must be a leakage. Likely places are hoses, and the water pump. If it is the water pump, the leakage may only be evident during engine operation, unless the leakage is severe in which case you will notice coolant dripping. Regards John

It's very possible it could be leaking and need replacing.
$500 is too much to pay for this repair.
I would get a second opinion and another cost estimate.

check ALL hose clips,if a pipe is "blown"the clip will not hold the pressure,hence leak.

check the leak if its coming from loose hose clips, bypass pipes, broken radiator, coolant reservoir or from the water plugs in your engine block..

those are plastic hoses one must have broken. hard to repair. sometimes its just the retaining clip is it leaking behind the fitting or right at the fitting. you might try pushing the line in toward filter maybe just not clipped on tight.

Age of vehicle determines that you get a replacement hose and clips first. Drain down the Coolant into a container, replace the hose, and refill, remembering you are going to loose coolant by spillage on the drain down.
Regards, DT

Have a mechanic pressure test your cooling system for leaks (or buy a tester and do it yourself). Check engine oil to make sure no water is getting into your oil due to a cracked head.

NO IT'S NOT NORMAL
u might have a loos in one of the coolant hoses