Crash pointed me to this, so I figured it might as well jump in because I've had some dealings with this type of thing too in the past.
First off, as you pointed out, you mad a mistake of not talking payment early on. If they said "yea, we'll pay you" then you need to get a range that you both of you agree on. Can't fix that now, really, but if you haven't given them the final map, don't. Not at least until you sort out a plan on what to do.
Secondly, sounds like you made a rookie mistake and let client feature creep kick in. This is generally pretty bad. When you set up a contract like this, you should always have a few things set in stone: What needs to be done, how long it can take, how payment will work, how much you'll be paid. If the client wants to add to what needs to be done (Either, they need you to make art assets, or they want a new feature, or they want more testing), you have to stop work, and renegotiate out all the other thing listed. Don't just pile stuff on and not talk about the other things. Ontop of it all, get it in writing. Ideally, you have a lawyer draft up an appropriate, legally binding contract, but people who pay 25 bucks for a map usually don't know lawyers exist for this reason. Generally, if you should a screenshot of whatever was agreed upon. (I'm not a lawyer) - This isn't legally binding, but it does provide a reference you can pull on to say "we agreed to these terms" Just incase disputes or situations like this pop up.
Thirdly, like mentioned before, you should probably hold onto the map for now. I wouldn't say take it hostage, but as of now, it's technically still yours. You can do whatever you want with it. Don't give them a final version of the map until this is settled. They want this map, so they should be able to fork over for it. Unfortunately, they may not want to pay your rate. In which case, they don't pay, they don't get the final map and you can just sit on it. It'll suck, and you'll take a loss, but it's better than just giving it to them. There's other things you could do if you wanted that would be a little more.... spiteful... but I won't get into them now.
Your next step is to basically say, as politely as possible "I don't feel that the payment you're offering is enough compensation for the amount of work that I put into this map." Re-enforce that they will not get the final map file until you and person hiring you come to an agreement on payment. If I had to guess, then they'll be like "okay, hold on, let me see if I can scrounge up a few more keys." TF2 trade mappers are notoriously bad at properly compensating mappers. You shouldn't expect pro rate, but you should at least be getting something that is respectful and not insulting. If they offer stuff like VIP on their server, or special mod powers or whatever, I personally would turn those down. They have no value. It's like getting paid in equity in the company. You want keys, or paypal, or unusuals. Something that you can definitely turn into something you can use. VIP on a server doesn't give you anything.
Don't accept a value until you're happy with it, and don't feel like a dick for charging more than they offer. Only getting 10 keys? You can add a 0 to the end of that if you want. Hell, maybe 2. It's whatever you feel the time and map are worth. You're not being a dick by asking or talking about it, you're being smart. You're being a freelancer. You're doing it right.
I can help out more if you need to, so feel free to ask here or DM me on the forums and I'll get back to asap.