There is no standard rate, it varies significantly from one processor to another and one merchant risk profile to another. Walmart (massive volume, card present=swiped, low fraud rate) probably secures custom deals for an interchange below 1%.
On the other hand an adult services company (low volume, card not present, high fraud rate) can end up paying 10% or more. Throw in the cost of anti-fraud screening, and the cost of chargebacks (getting hit with $35 chargeback fee when your average purchase is <$35 is going to hurt your bottom line), the true cost can approach 15% or more.
There's also the fixed fee many apply, either in the form of membership/subscription fees or possibly fixed-cost fees (usually called "transaction fees") added on top of the percentage-based fee. You can factor in the cost of hardware and training, too. It's hard to determine an "average" without a shit-load of data and a team of people going through it. You're probably looking at 2.5% on the very low side, 20%+ on the very high side if a merchant deals in low-value transactions (US). I'd guess it probably averages to around 4-6% after all's said and done (if not, higher).
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