The min spec for K100RS4V/K1/K1100 front rotors is 4.50mm.  From my experience you'll hit this somewhere around 50-60K miles. As you'd expect, buying new replacement rotors from a BMW dealer is insanely expensive and as of today the price for a pair is $1,043.94.
 
Motobins sells Brembo complete replacement rotors for £125 each.(without VAT) The Motobins site says they're Grimeca rotors but they come in Brembo boxes and they can be found online under the Brembo part number B408.46. Although they are "complete" rotors with both carrier and rotor you'll need to re-use the ABS ring from your old BMW rotors.
 
Here's what they look like installed:

 
 
James Sherlock also sells replacement rotors. These are just the rotors though (no carriers) and are Hekkane part 011904.  Sherlock lists them on ebay but you can get a better price if you order them from the James Sherlock website where the price is £44 each. (without VAT)
 
Since these are just the rotors you'll need to re-use your factory carriers and probably want to use new bobbins as well.  Both Motobins and Sherlock sell bobbins that are stainless steel and probably hold up much better than the aluminum alloy OEM bobbins. And of course they cost a lot less than BMW bobbins.
 
My personal preference is the rotors from Sherlock since they have angled ventilation holes like the OEM rotors but I don't think you could go wrong using the Brembo rotors from Motobins which will be easier to install since they're complete with the rotors already mounted on carriers.
 
Note that the Sherlock site specifically states that they don't ship to the US but I decided to try anyhow and after purchasing the rotors they sent me a PayPal invoice for £27.60 to cover the shipping cost. After I paid the shipping (via DHL) it only took five days for them to get to me in the US.  Motobins usually takes about seven days via Royal Mail so delivery time is roughly the same.
 
I have not installed the Hekkane rotors from Sherlock yet but here's what they look like naked:

The ventilation holes might be different, but I think I would go with the Brembo rotors. Less faffing around and they are Brembo.

The brakes are Brembo and its a fair bet Brembo actually make the 'BMW' rotors anyway.

Just like buying a 'BMW' clutch plate...made by Sachs....

Those are stunning price differences!  Am I the only person who finds those margins offensive?

Finding the margins on OEM parts offensive is the first part...

...Finding alternatives is the second.

When cheap parts crop up it can be a smart move to buy them. Especially the hard to get parts like lenses, lights, switches and even body panels.

Regards Martin.

MartinW wrote:When cheap parts crop up it can be a smart move to buy them.

Is why I have two sets of cast-iron rotors in the cupboard... too cheap to resist... And that full set of Alaska Blue bodywork I just bought... and... and... Dammit, the garage roof is almost full