I think that price limits would not be very good. I'd worry about availability and creation of second-hand market, where third parties would buy out the supplie

I think that price limits would not be very good. I'd worry about availability and creation of second-hand market, where third parties would buy out the supplies, to resell them later at higher price, before the manufacturer manages to re-stock the spare parts supplies. But I think mandating that replaceable parts have to be standardized, would make it so that multiple manufacturers would be using compatible parts. As a consumer/repairman you'd be able to choose which one to buy from. This would introduce competition that would put the pressure on the price. There would possibly be even less waste because standard parts could be more easilly reused with any fitting brand/manufacturer of the product.