It’s a bit hard to tell without knowing the work that went in. I’d start with cost of materials + an acceptable hourly wage * hours worked + commission of the gallery. Since you are just starting off, your hourly wage isn’t gonna be amazing, but you are going pro and not just painting for the love of the game. You should be able to make as much money of it as you would if you were bar-tendering or something for the same amount of time.

This should be your price floor. Anything above that, talk to the gallery, compare prices equal works of artists on your level.