About Giving price to your work

Hello! il start up saying English is not my native idiom so apologies in advance for my bad grammar or if i don't explain so well (if u score my English 1 to 10 at the end il appreciate it).

After finishing the design course and knowing something about html and css i decide to start with my own project helping a cousin of mine making his webpage for his restaurant.

Thinks goes smoothly and he really liked the design and the future solutions the page are giving to him, after a meeting he insisted to give me a royalty (at the beginning i was just making his webpage to start with my portfolio) and i rejected but he seriously insisted.

Now here is where the problem begin.. i am seriously bad at pricing my works and if things goes well i might end up with the same problem with future works for people that are not relatives and they might misunderstand my quality or take advantage of the low pricing.

Is there a track that help out to price works? or/and at your own experience how did u learned to price your works?

I live in Mexico so if someone know about pricing works where i live il be more grateful, its ok if pricing come from your country i just want to give myself an idea.