I'm giving a suggesting, and the coin system is a valid complaint with the game. HMX isn't the perfect game developer. No one is. I'm suggesting that they tweak the restarts because that's where the coin imbalance lies. Restarts takes way too much a loss and hurt the game. They loss you get from restarting add up very fast.
So this is how it should happen.
Restarts are Free or no more than 50 coins except, when you restart you must use the exact same powerups. If you change powerups you have to pay for that powerup again. If you go to Quit Song, or go to a different song you have to pay again. Or at least make restarts with the same loadout something like 50 coins.
Or another suggestion is to make Restarts cost 1/2 of the coins it normally costs on a song, and make it so you get more than 50 coins for GSing a song. 150-200+ profit for GSing a song is alot more reasonable. You barely get any coins if you are not sightreading as it is right now. Sorry but not everyone has an endless supply of unplayed DLC songs to sightread and get 2x coin bonus. People are taking the 2x bonus for granted because you won't be getting many additional coins once you play all your songs at least once.
The people who never restart or have a truckload of coins won't be affected by this at all, however for the rest of us who don't have many songs to SR, and can't do song specific goals because not having the said DLC song or want to use facebook can restart songs without being completely bankrupt all the time.