Well, Watanuki could certainly rationalize that he certainly shouldn't tell Doumeki about it right then and there. And if it 'slips' his mind, or there's never a good moment -- well, it's not the same as lying to Doumeki, right?

Right?

It might not hurt to make Doumeki's suspicions about prices explicit -- it might be a good way to introduce the first mention of his grandfather, if you wanted to set that up.