First, if doing the escrow through Sedo, you can read their
price list here. Fees differ according to how the deal is done. If the domain is listed/parked with Sedo, the fees may be different than if you're using them just for the escrow but domain is not listed there. Read down the price list to see which price applies to your exact situation.
In your case, where the domain is not listed at Sedo, and you and the seller have come to a private agreement and now only wish to use Sedo for the escrow transaction, you can see the fee/s for that in the above list I linked to.
As for paying the commission: since you and the seller did not discuss this when you arrived at the sales price, this is yet another negotiation, separate from the original price negotiation. Talk about this and reach your decision with the seller privately, before you bring the transaction to Sedo for escrow. If you were buying a domain that was listed on Sedo, you would hit the BIN button (or make an offer) and the commission would by default be paid by the seller (unless you went through a Sedo agent and specified that you wanted to pay the commission - this sometimes happens when you wish to make an offer for the domain).
Before talking about this privately with the seller, you would have to decide, looking at Sedo's fees, whether the extra commission fee is a deal breaker for you, or not. If it's a deal breaker, you can tell the seller that if you have to pay the commission, the deal is ended. If the seller doesn't mind paying the commish and is happy with the sales price after commish, that makes it easy.
If you can budget half the commish, that gives you more leeway for negotiation. If you can budget the whole commish, then there's no worry; you can try get the seller to pay commish, or to pay half, but if they stand firm and will only sell the domain if they can get full asking price and you pay commish, then at least you know that your fallback position will still get you the domain.
Basically boils down to what you want to budget for this domain, versus what the seller is willling to accept for this domain. Sometimes the question of who pays commission does break a domain deal. But that is rare; the commish is a small percentage of the selling price and if the buyer really wants a domain, they **usually but not always** are fine with paying the commish fee too, if it is necessary.