Why 41¢ isn’t always 41¢: the spread and the order book

Open any market on Polymarket and you see one number — say 41¢ for YES. That number is real, but it only tells you the price of the cheapest share for sale. What you actually pay depends on how much you buy.

Bid, ask and spread

• Ask — the lowest price someone is willing to sell at. This is what you pay when you buy right now.
• Bid — the highest price someone is willing to buy at. This is what you get if you sell right now.
• Spread — the gap between the two. If YES is 40¢ bid / 41¢ ask, the spread is 1¢.

The spread is a cost. Buy at 41¢ and sell a minute later and you get 40¢ back — you lost 1¢ a share without the market moving. On busy markets the spread is a cent; on quiet ones it can be five or more.

Depth: how much is for sale at each price

Behind the best ask sits a queue of other sell orders at higher prices. Here is a simple example of the YES side:

• 500 shares at 41¢ ($205)
• 1,000 shares at 43¢ ($430)
• 3,000 shares at 46¢ ($1,380)

Buy $100 and everything fills at 41¢. Buy $600 and you take all the 41¢ shares, then most of the 43¢ ones — your average comes out near 42.3¢. Buy $2,000 and you are deep into the 46¢ orders, averaging about 44.8¢.

Why this decides whether an edge is real

Say you believe YES is worth 45%. At 41¢ that looks like a 4-point edge. But in the book above the edge only exists while you buy below 45¢ — roughly the first $635. Everything you buy beyond that is at or above fair value. The bigger your order, the less of the edge is left.

This is why a small gap on a thin market is often worth nothing, and why the same gap on a deep market can be worth a lot.

Three habits that help

• Check the spread first. If it is wider than the edge you think you have, walk away.
• Size to the book, not to your conviction. Know the price you will average before you click.
• Use limit orders when you are not in a hurry. Posting a bid at your price instead of taking the ask saves the spread — at the risk of not getting filled.

Every open DeeplyticAI signal has a Before you trade block that reads the live Polymarket order book for you: the best price and spread, your average fill for $100, $500 and $2,000, and how far the edge holds before the price catches up with our estimate.

Analysis, not financial advice.