Lifetime entry
The Grand Rapids Marathon, in Grand Rapids MI, is offering a "lifetime entry," allowing runners to race in either the full or half-marathon for the rest of their lives with their own, permanent race number.
The lifetime entry is $650. Current marathon entry fee is $67 and the half- is $56.
For a runner who expects to race for many years (that's what most runners expect), it's a great deal. With entry fees increasing each year, the lifetime entry could pay for at least nine more events. Every race after that would, essentially, be free.
Presumably, race organizers have calculated this offer and determined it to be profitable for the race. The upfront payment would provide additional cash (that could be invested) and all lifetime entrants would not necessarily run in all the races they've paid for. Injuries, personal schedules, health conditions, and running interest could change over time.
If you think something like this might be a good idea for your event, talk to the Grand Rapids Marathon, and then think this through very carefully.