This is not intended to be rude at all but you have to realize something. It takes an incredibly long time to make a map of something like a city or a town or even a province for a country. Maps are not like art on Deviant Art. On deviant art I have seen people sell their product for five dollars. You won't find that kind of thing here. People on this site do this for a living and need to feed their families. And maps take a very long time to make. This means that the person making the map has to take all of that time to make your map for you. They could be out working and making money to feed their family. But if they have to devote time to making a map, that is money they are not making, and time being spent. How else would they be able to feed their family? So they have to make it worthwhile.
In all honesty you are asking the map makers to make you something that you might publish, that you might make a lot of money on, and you want them to sell you the rights to make money on it, but keep the map maker from making any money on it by restricting them to only use it in their portfolio and insisting they do nothing with it commercially. You must realize that this sort of thing costs a lot of money don't you? The map makers here are not going to sell you a map for dozens of dollars. Try a few hundred per map or more. This is more realistic, especially considering what kind of rights you are asking them to sell you.
Again, don't take this the wrong way, but a lot of people always approach artists like this. They seem to think that by having the artist make them something and paying the artist little, but promising the artist exposure by having the artist able to sign his art or put it in his portfolio, that somehow this makes up for not paying the artist. It doesn't. Unless a person is a representative of a publishing company that is well known, and makes the artist a promise that their art will get a lot of exposure using the publishing companies media, exposure means nothing. It gets the artist nowhere.
Now I know you didn't specifically mention exposure but it did sound implied by saying the artist can sign his work. This made it sound like you are saying that somehow, the artist signing his work will give him exposure.
I'm sorry if this sounds rude but it wasn't meant to be. It is only meant to help you see the reality of the situation.