Boeing may well say $5m in spares is recommended. No operator in their right mind would have that amount of spares per aircraft.
Try around $1m in spares for a single 73 - there aren't many places on the planet you can't arrange a loan at short notice for a 73 part (and at worst, just cop the AOG and fly the part in). And if you have a reasonable-size fleet, no need to have even $1m per A/C.
Don't forget, Boeing sell parts and make money from them. So their recommendations in this area should be taken with a pinch of salt. Economics dictates taking a bit of downtime or copping loan charges rather than having lots of little-used, expensive spares sitting around not doing much.