His lips find their customary pout, as they always do when he's thinking, or feeling particularly bullish. This is one of the former expressions.

'An' there was me thinkin' you were doin' it out of th' goodness of your heart.'

It sounds vaguely mocking but it's himself it's aimed at and hell, he's not about to complain about her earning a living. He just already feels like a bit of a twat up here, even though it's something he's always wanted to learn, and needs something to take his mind off it.

'Shoul' charge you for drivin' lessons.'