S&P 500 (SPX) Up or Down on July 31?

Platform comparison

Market context

The S&P 500 is ending the session with the index higher than the previous close, which means the contract is already on track to resolve **Up** if the official closing print stands. The latest available close before today was 7,437.63 on 30 July, and Friday’s market report has the S&P 500 finishing at 7,489.72, a gain of 52.09 points, or 0.7%[3][2].

The current crowd-implied probability of **100% YES** leaves essentially no room for disagreement in the market data, and it is consistent with the fact that the index has already posted a positive daily move. That makes this a poor place to look for genuine pricing divergence between prediction markets and any sports-betting analogue; there are no sportsbook lines for a cash equity index close in the usual sense, so the relevant comparison is really between crowd pricing and the realised market outcome. The broader backdrop has been volatile, with the S&P 500 recovering sharply after the Fed held rates steady on Thursday, following a heavy sell-off the prior day[1][4].

For traders watching the last dependencies, the only material issue is whether the official closing value used for settlement matches the standard S&P 500 close and not an intraday estimate. Since the event settles on the final July 31 closing print, late-session rebalancing, headline risk, or after-hours moves do not matter unless they change the official close itself. The day’s rebound was led by tech and consumer discretionary shares, which also explains why the index has been able to finish above the prior session even after a weak stretch in the broader market[1][2].

Sources: 1 · 2 · 3 · 4 · 5

Methodology

Methodologically we separate two layers: the live probability (Polymarket mid-price) and the platform attributes (fee, KYC, settlement currency, payment rails). That keeps the comparison honest — a single canonical probability across the row, with the venue-by-venue trade-offs spelt out in the columns next to it.

Resolution & payout

Polymarket-based markets settle through the UMA Optimistic Oracle on Polygon. A proposer submits the outcome, a two-hour challenge window opens, and unchallenged proposals finalise the resolution. Payouts settle automatically in USDC the moment the result is final — no bookmaker, no delay.

Kalshi-based markets settle in USD via the CFTC-regulated clearinghouse. Betfair Exchange settles in GBP/EUR net of commission. Manifold is play-money and does not pay out real funds.

FAQ

Where can I trade this market with the lowest fees?

Polymarket is geo-blocked in the US/UK/EU. The easiest 0%-fee broker into the same order book is Kalshi vs Polymarket. Kalshi charges up to 7% per trade; Betfair Exchange takes 2-5% commission on net winnings.

What's the difference between YES and NO shares?

A YES share pays $1.00 if the event happens, $0 otherwise. A NO share pays $1.00 if the event doesn't happen. The market price between 0¢ and 100¢ is the implied probability.

What does Polymarket cost to trade?

Polymarket itself charges 0% — the only cost is the Polygon network fee, typically under $0.01 per transaction. Off-chain venues like Kalshi or Betfair charge 2-7% commission.

How fast are USDC deposits?

Polygon credits deposits after 12 confirmations — usually under 30 seconds. Withdrawals follow the same path and land back in your wallet within minutes.

Do I need to KYC for this market?

On Polymarket directly, no — it's wallet-based. Intermediary brokers like Kalshi vs Polymarket trigger KYC only above $1,500 of lifetime trading volume; under that you trade pseudonymously with a single wallet address.