Hi,
Unfortunately I bring bad news - dev's estimate was higher than yours and, as he said, his primary focus is the development of the game at this point.
He measured the value of his time by the number of people who are looking forward to seeing the updates to the game, so the commission's price should somehow reflect the amount of time taken away from the development, as this is his actual irl work and the only source of income.
He didn't decline it completely though, maybe much later he might change his mind regarding commissions, depending on the number of people asking for them, however keep in mind there's nothing set in stone.
Cheers