I just bought a couple of tickets to a small venue, tickets were $26 each (NOK is weak these days) including a fee of $2.25 each, so grand total $52 of which $4.50 total in fees. Checking my card, this is indeed what my card was charged, so no additional hidden fees.
I bought some tickets earlier this year for a mid-sized venue, $44 per ticket including $3.50 in fees each.
I did not select the optional "services" like physical ticket (just eTicket which is e-mail with PDF) or the ticket insurance.
Checking my ticket history (~10 concerts through Ticketmaster last couple of years) it's always been around this 10% in fees. It's comparable to the other ticket vendors other venues use that I've bought from like TicketCo and Tikkio.
edit: I checked Bryan Adams, he's coming to a larger venue next year which also uses Ticketmaster. Only tickets left were the least expensive ones (standing) at $75 per ticket, including a $5 fee.
edit2: Besides Ticketmaster not owning venues as mentioned by sibling reply, I guess a big factor is that, AFAIK, any non-optional fees must be included in the "sticker price" here in Norway. So they can't tack those on later.