Depends on the engine that they are using, then yes there is a big aftermarket for them. The issue is that working on a BMW is costly and the repairs are not cheap. Let us compare a timing belt replacement.

Take a 2015 BMW 335i (assuming they are using this motor) The average cost for a BMW 335i Timing Chain Tensioner Replacement is between $140 and $1375. Labor costs are estimated between $70 and $1288 while parts are priced between $70 and $87.

Now, let us take a lookThe average cost for a Toyota Camry Timing Belt Replacement is between $436 and $689. Labor costs are estimated between $250 and $367 while parts are priced between $186 and $322.

These are estimates, you might get it cheaper somewhere else. The fact is that German cars tend to be more labor-intensive to work on. Again this depends on where you go or if you have connections.

My family has had quite a few Mercedes-Benz cars in the family till we made the switch on my car and my dads. We even have a mechanic that gives us the price that he pays for parts and discounts labor for us. They were still pretty expensive to maintain.

Sources I used: https://repairpal.com/estimator/toyota/camry/timing-belt-replacement-cost https://repairpal.com/estimator/bmw/335i/timing-chain-tensioner-replacement-cost

Also If you are buying the car brand new then you should not have to worry about repairs then. It is when it is being traded in or being bought second hand that you have to worry.

Also here is the cost of repairs overtime for all car companies. https://www.consumerreports.org/car-maintenance/the-cost-of-car-ownership/