This is always a complicated thing. And it's that way for everyone who does it until it becomes familiar.
Some things that differ here - with many companies in the game industry, they have an amount that they are shooting for.
They won't tell you this amount. You have to guess, in a sense, what they may be willing to pay. Or you have to determine how much you would do it for.
There are some generalizations regarding prices in the games market - like what are averages and such.
They may pay more for one person, because they really want that style of map, and less for another.
But, they may not pay more, even for really awesome work - they have a fixed amount in their mind and won't budge.
It all depends on that particular client.
Each of us has been turned down by various people because they don't want to pay a certain amount.
Sometimes that amount is a decent and fair cost for the amount of work.
Evenso, some still won't go above a certain amount, even if that amount is not really a fair price for the amount of work.
This is what we all negotiate with all the time. Some will underbid, just to get the work and the client.
That is actually counter-productive in my opinion. It also keeps the ceiling low for everyone else, but that's another matter entirely.
Some people are faster and can do a certain amount of work in a shorter time, and as such can charge less and get the client.
There is a lot that goes into it all. It's not an easy formula and as such there's no set answer I can give.
If a map takes you ten hours, and you want $10 per hour, then the map would be $100.
If you want $20, then it's $200. That is one way to do it.
But what if that map is going to take you 50 hours.... will a client accept you charging them $500-1000? Probably not.
But I might be able to do that piece in 20 hours and still charge them $500-1000, and I might get it. Depends on whether they really want my work or not.
I've often found, that when a client just wants any map maker to fill a job that they will usually be low on cost and won't pay well.
When a client really wants a certain style or a specific artist, they usually pay better because they are after something specific.
Others may have some more input they can give you. I hope some will respond. This has always been so arcane and secret.
It is hard for people to learn how to proceed if no one will tell them anything.
In any negotiation, always remember to respect yourself and your time and abilities.
If a dollar amount is insulting for the amount of work, it's okay to be insulted.
Illustration rates haven't improved much in 30-40 years.
It's always a struggle until you're well known and more importantly, well connected.
If you have any more specific questions, I can try to answer them.
There's no risk in trying either. Think of a price you'd like to make for the maps and give it a shot.
It always starts with a risk and a step forward.