This article is about the challenge. For the item, see Pay to Play.
PAY TO PLAY is Challenge #24 and was added in The Binding of Isaac: Afterbirth. In this challenge, Isaac starts with 50 coins, Money = Power, and the Sack of Pennies. However, every doorway is now an arcade door, and taking any damage will cause Isaac to lose more pennies than can be picked up, similar to when being attacked by Greed. Having 0 pennies will kill him and end the challenge. The goal is to defeat Isaac. This challenge is unlocked by unlocking the Sack of Pennies, done by defeating Isaac with Cain.
Needing money to continue the run causes shop purchases to become much riskier, and extra coins may have to be found in addition to hearts when getting hurt.
The arcade doors in this challenge appear to be "overlaid" on the level's normal doors, even after they have been opened. In most cases, this allows Isaac to unlock and enter rooms that normally require keys (or bombs) with a coin instead. However, certain doors are still "active", and can still damage or block the player, even after being unlocked with a coin. These doors will appear to be normal open doors, despite the effects triggering when attempting to pass through them.
Curse Room doorways will still damage the player (without flight and/or shielding) after unlocking them with a coin, despite the absence of visible teeth around the door frame.
Challenge Room and Boss Challenge Room doorways will still prevent the player from entering with too few or too many red hearts respectively, even after being unlocked with a coin and appearing to be an open doorway. Unlocking a door for which Isaac has too few red hearts effectively wastes a coin (unless it is planned to return with the right number of red hearts).
Also, the doors to these rooms will appear to be the same as all the other doors on the level. It may be helpful to keep an eye on the mini-map to ensure that health (and coins) aren't needlessly lost to a Curse Room, or waste a coin on a Challenge Room he cannot enter. (Bombing into these rooms appears to leave them unaffected.)
Presumably because they hold 0 coins Co-op babies are killed instantly upon entering the game.
A Quarter and A Dollar are fantastic items in this run, as they give Isaac a good amount of coins. IV - The Emperor card is useful in this challenge as it teleports him directly to the boss, preventing the loss of any coins.
Lucky Rock will allow Isaac to gain more coins simply by destroying rocks. Combined with a means of infinite bombs or any other way to break rocks without a cost (such as Leo), this trinket becomes invaluable.
Counterfeit Penny can increase the value of found pennies. This also means that when damage causes Isaac to drop coins, it is likely to have a net gain once collected. If lucky, this can result in nearly indefinite coins, making this challenge mostly trivial so long as Isaac has a steady health supply.
Midas' Touch can be used to turn enemies into gold that splits into coins.
Glowing Hour Glass can allow Isaac to exit dead-end rooms, earning him back the coin spent getting in.
BFFS! gives Sack of Pennies a 33% chance to drop a coin in rooms where it normally wouldn't.
If the player has many bombs to spare or a renewable source of explosives (such as Epic Fetus), bombing through and out of Secret Rooms can save some coins. Since rooms count as "unlocked" on all sides once they have been successfully entered and cleared of enemies (except for the Shop, which must be exited through the main door to fully unlock), the remainder of the rooms bordering the Secret Room can be unlocked with bombs/explosives instead of coins.
Items like Treasure Map and The Compass can help narrow down where Isaac can go and where is likely to just be a dead-end, so they can be valuable purchases from the Shop. The boss room usually appears at a dead-end far away from the spawn point.
Shops usually appear farther from the spawn point than Libraries. Both can contain useful items, but the Shop generally has better items for this challenge.
Additionally, Guppy's Tail and Mom's Key can provide a sustainable amount of money given that each chest opened will likely contain a large amount of coins and other consumables.
A combination of Broken Remote with active items that are always charged (or have a recharge time in seconds) can prevent the loss of coins to doors, which can prove to be very effective, but may cause wasted time trying to reach the room desired.
When exiting Curse Rooms, it's possible to quickly backtrack into the room to pick up the coins that dropped due to taking damage from the door frame, and then quickly exit once again, without taking damage for the second time. This allows you to conserve coins, and is possible to pull off even with Isaac's starting speed.
Blood Oath can kill Isaac by rapidly draining all of his coins.