Each agent will determine their own price to issue and service a RTW ticket for you, it's up to them what they think it's worth based on the amount of work involved in them, especially if you're new to RTWs and likely to have a lot more questions and require guidance more than others who know the rules already and do their own planning. I've been told that the commission from the airline isn't very much on these tickets, so I'd anticipate all agents to have some fee on it, but it should be well worth it depending on which OW airline you end up ticketing with. Some are very hard to work with (QF for example), some don't issue their own RTW tickets (QR for example), some charge their own fee every time, even for otherwise free changes (JL). An agent can also help you optimize for YQ / YR (fuel surcharges) by using certain carriers to ticket on and certain carriers codes on long haul flights, that can easily make a difference of $1000 on a ticket.

To get started though, you will need to decide what your priorities are for your ticket. Do you want to maximize mileage or status point earnings in a particular program? Do you want to see exotic places that are expensive to get to on normal tickets? Do you want to spend a bunch of time in the top premium cabins in the world? Or just get a good deal for a bunch of flights that you might need to take anyway, and would certainly be more expensive booked a la carte. (None of those are mutually exclusive, just something to consider when starting your planning)