How quickly can you do the work, approximately? Multiply that time by your desired hourly rate, which should probably be upwards of $15. The better your work and the quicker you can produce it, the higher an hourly rate you can charge, but $15/hr is a good starting point. If you live in a country with low costs of living, you could perhaps go for a lower hourly rate, but you shouldn't need to. Clients unwilling or unable to pay a reasonable rate for fully custom pixel art have other, cheaper avenues available to them.

There's no good formula for calculating a suitable hourly wage. The main factors are 1. how much people who actually want YOUR* work are willing to pay for it, 2. how much your time is worth to you. If you're not desperate for money (e.g. have a day job) and do pixel art primarily for fun, then the second factor should be the main one.

There will always be people trying to lowball you, and unless you're desperate for money, they're not worth the effort of healing with them.

* I specifically bring up "your work" as opposed to "just anything that does the job" because for custom art, the good clients are typically there for your style and skills. If they wanted "just anything", they wouldn't need fully custom art. Some clients don't understand this and seek to pay stock art-like prices for custom art. Usually, just explaining the difference is enough. When it's not, they're usually not worth the trouble xP