The $1 isn't helpful if you get it on another player's turn. In two-player games, that makes winning the split a giant economy swing (you play 6 Marathons on your turn and get $6, also triggering your opponent's four Marathons for $4 which is useless to them).
I don't anticipate there being many boards where playing 5 or more Marathons per turn is a viable strategy. Your deck would have to cope with that many Marathons in addition to all the weak cards being gained by them; as well as drawing your deck each turn, you'd want something like Watchtower, Donate, Highway, or two spammable $2 piles on the board.
When such a deck is possible, winning the Marathon split is obviously very good, but mostly because you get 4VP per turn more than your opponent, rather than the economy boost.
I hope more games would be about one player choosing to add Marathons to their deck even though they don't expect to play 5 per turn. Maybe they intend to gain and play their 5th Marathon just before the game ends. The other player(s) could: (i) compete for Marathons, probably leading to a rush/slog in which the +$1 adds interest, rewarding the finisher each time; (ii) avoid Marathons, effectively forcing the Marathon player to get at least 5 of them; (iii) mostly avoid Marathons, but cheekily gain and play the 5th Marathon to rob the Marathon player of their anticipated $ and flood them with 4 stop cards.