According to Edward B Westermann in his book, FLAK German Anti Aircraft Defenses 1914-45, University Of Kansas Press, 2001, the average 88mm ammunition expenditure per kill over the Reich in 1944, was 16000 shells per kill, At approximately 80 RM per shell, that equates to RM 1280000 or $512000 per shoot down. Viewed in those terms, the Flak arm was a most inefficient method of air defence.
By comparison, the US Navy is estimated to have been expending just 550 rounds per kill in the Pacific. This figure applied to the end of 1944. At the beginning of 1944, it was taking about 1500 rounds per kill.
The two figures are not exactly comparable. Whereas the Germans were firing at high altitude targets, which were above the effective ceilings of the 88mm guns being used by the Germans, the Japanese were attacking at low level, using ordinance that required them to fly low, straight and along a very predictable path of approach.
Still, whereas the the US/Allied AA effectiveness was getting better and better as the war progressed, the Germans were getting worse and worse.
Barrel wear in the AA park and the decreasing standards in crew training were having a catastrophic effect on German AA. The effective ceiling of German AA, according to Westermann had dropped to about 24500 ft by mid 1944, because the barrels were so worn and the guns were operating way below spec. The number of burst barrels had increased from about 20 per month in 1942, to well over 300 per month in 1944.
The poor level of crew training was such that the Germans were basically reduced to barrage fire over aimed fire in 1944. Whereas the majority of flak troops were regular army in 1942, by 1944 they were mostly landswehr...part timers, lacking in the training to be anything but a scare factor in the strategic bombing campaign. They had essentially traded places with the British Flak troops of 1940. in 1940 the British had basically been able to scare the Germans with their AA fire effectiveness. By 1944, it was a very efficient and well equipped force.
A measure of just how badly German effectiveness had sunk is the fact that whilst it was taking 16000 rounds to bring down an Allied heavy bomber in 1944, in 1942 it was only taking the expenditure of about 4000 rounds per kill.
Mind you, achieving a kill by AA is about the least important function of flak. Its most important job was to disrupt the accuracy of the bombers, and to keep them high, and thereby also decrease accuracy. In this area, the Germans were effective until the end of the war