The income or wealth does play a part in it but that the German system always allowed deferred payments (or payment in rates) but it is not the factor deciding against it except if the offender is absolutely incapable of doing it which means usually either added prison time, or in milder cases (speeding etc.): communal service. The difference is purpose: It serves as compensation and satisfaction. Whereas in some countries the purpose is to get rich. Here it is based on the material damages and pains caused. How much is to be paid is based on facts of the case and evidence. There are also reference catalogs being used which in total would give a a mathematically determined total sum which then is negotiated in court however it can't get under 20% of that without risking an appeal.
Basically 43,000 € may seem absurdly low according to American standards where it would be 1000 times higher, in fact there have been high-profile cases in Germany where the sum has been as more than ten times as high but this never has been the underlying philosophy here. However a lot people misunderstand why this happened. The court could have opted for no fine and higher time (given he will end up in preventive custody it wouldn't have mattered by much). I think the intention was to satisfy the damaged party as well. The kid will never see these people again, no money will repair that trauma but it will at least give him and his caretakers a realistic head start for a new period life.