What is a reasonable charge to replace a radiator in a 2007, 335i?

We don't get involved with costs here because there's too many variables. From recent experience, a BMW brake rotor costs close to four times as much as one for a Dodge Viper. BMW is very customer unfriendly when it comes to replacement parts and service information. They don't release paint codes and service information to other companies so you have to buy stuff right from the dealer and pay whatever they want to charge, ... And they're pretty proud of their parts and charge accordingly.

You might want to get an estimate from a radiator specialty shop. If they have a source for the right parts and mounting brackets, they might be able to produce one for your car, but usually they just order the right replacement from aftermarket suppliers. Those suppliers are often the same ones that supply the original manufacturer.