Hi Mexikorn,
I think its a good thing that your asking about it instead of just wading into the melee that is commissions. First of all I would say do some free ones first and then you can get a feel for what people want from commissions in general. Also, the free requests, by their nature, tend to be less restrictive and more lenient with the requirements. Also, with the free ones you can bail on it if your not getting on well with it and also the client will have to accept whatever you put out since there is no contract. Speaking of contracts its a good idea to do the free ones and get a feel for what to do before having contracts written enforcing what you must do to satisfy the contractual obligations. A free commission has no obligations. You as the artist are also free to do what you like with the art you create without fear of offense or copyright (for the most part). Of course some people might take offense at your free work but that's their problem. When its free you get what you paid for it.
Having a selection of results from working for free commissions gives you a gallery of maps to show the standard of your work. When a client requests a map they might select based on your style instead of just price and quality. I always strongly recommend that people requesting a commission for money look at resulting previous commissions from an artists *on this guild* instead of other sites. Some people can take others work for their own and claim to be great and run off with payments. Onus is very much on the individuals to vet the artists and due their own due diligence. So good to have that built up as a reputation of historical completed commissions from the commission requests through to the final delivered maps.
Also bear in mind that some people on the guild are awfully good and if your client is making a proper product with a good budget behind it and needs great artwork for it then they have a wide range of choice to pick the best. Thus naturally, some members here are going to get a lot more commissions than others. I say this so as not to get any discouragement and it may take time to be competitive in the market. Those great artists here are likely to be setting a fee which is higher and also may have a wealth of work lined up with little free time to do any more which differentiate you and is where price and availability might mean your successful. So its all about where you place yourself and to know that information means you need to get in there and work at it a bit to get a feel for it.