I would typically charge based primarily on the amount of time it would take me to make it, with a flat rate per hour, then quote based off that. How much you can charge per hour depends upon your style, skill and speed, and pretty much what you think you can get away with If you value your time at $15 an hour and it takes you 2 hours to design a single map then you need to ask yourself if someone is really willing to spend $30 on a single roomed map, balanced out with if $5 is more appropriate for a single room and it takes you 2 hours to make that same map.. is it worth spending 2 hours just to make $5? If you are having to design unique assets every time, as opposed to recycling the ones you have made (furniture/trees/structures) then this is going to take a while, if they ARE fine you using pre-made assets then your speed will be a hell of a lot quicker.
Most of all you have to consider your customer and why they are buying "one offs" and what the end purpose is.
If they are buying the maps to publish encounters and are going to be reselling your (their) map, then you have some flexibility and probably could go more down the route of sticking to your price per hour if they feel they will adequately make their money back through their own sales. If they're selling single room encounters for $5 and your map design cost them $30 then they need to know how many units they are likely to sell because they will want to make some money themselves, not sell 6 units to pay off the cartographer and THEN start making money themselves.
If it's for their own personal use, ie. you are providing a service where you will draw DMs maps for them based off their descriptions, then they're probably only going to get one use out of it and wont want to spend that much. If you check out sites like RPGNow.com ad DriveThruRPG.com then you can get an idea of how much people are typically willing to pay for a single map of a room (probably $3 max for a single room), but these aren't bespoke maps so you can expect to bump the price a little. That said... who knows, some people might be willing to shell out more, tabletop players shell out a hell of lot for their hobby. If they ARE for personal use as outlined here, then maybe charge less but under the agreement that you are also allowed to resell the maps yourself afterwards, so you can at least try and make a little extra cash from them to mitigate a lower initial quote.
That might not be an exact figure as you may have liked, but truth be told everyone here will charge something different so you wont get the same answer. Some people just love making maps and are happy for a little extra cash on the back of something they already love doing, I personally run a D&D campaign so the maps I've started to design for resale are the ones I'm already designing to use in my own game. Other people wouldn't put pen to tablet for less than $100 and really there's far fewer battle map designers in this community than there are regioan/world mappers. Either way, I hope all that helps.