Pull a symbol's delayed bid and ask from this site's quote feed, or type them yourself, and see exactly what crossing the spread costs.
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The bid is the best price someone will pay right now, the ask the best price someone will sell for. From those:
spread = ask − bid · mid = (ask + bid) ÷ 2 · spread % = spread ÷ mid × 100
Buying at the ask and valuing the position at the mid costs ask − mid per share, which is half the spread; buying at the ask and selling at the bid costs the full spread. Both are shown per share and per 100 shares.
Bid $100.00, ask $100.20: spread $0.20, mid $100.10, spread 0.20% of mid. Crossing once costs $0.10 a share, $10.00 per 100 shares; a round trip through the spread costs $0.20 a share, $20.00 per 100 shares. Traded weekly, that drag becomes a fee you never see on a statement.
Only the top of the book. An order larger than the size displayed at those prices walks the book and pays worse than this says; partial fills, commissions, fees, rebates and slippage are not modelled. Spreads are not stable either — they widen in small and thinly traded names, in the first and last minutes of the session, and around news or halts, because market makers take inventory risk. The mid is a reference point, not a price anyone will trade with you at, and feed prices are delayed by roughly 15 minutes.
Arithmetic on delayed public data. No view on the symbol, no knowledge of your account or tax position, and no judgement about whether a trade is worth its transaction cost. Informational only.