'ni li pakala mi.' Ah, you wanted 'pona pona' for a superlative. Nice idea, which should get some traction, but tp doesn't have either comparatives or superlatives (well, I use 'pona ali' "totally good" for the latter, but that hasn't gotten any support). the trick of comparatives is y is xer than z is 'y li x mute. z li x lili' So superlatives are y is the xest = 'y li x mute. ante ali li x lili'
I tend to forget the 'mute' and 'ali' numbers for just the reason I just demonstrated. We haven't worked out how to do these cases either. You say "a monetary twenty", which works fine, but 'twenty moneys' (assuming mani is a fixed unit) would work, too. We need, as I said, to sort this stuff out.
I guess the 'mani', wherever it goes, can be understood, as is the notion that a thing IS its price.
I agree that we need, if not rules, then at least models for verbal behavior in various situations. tp doesn't really have them, beyond the generic 'toki' and 'mi tawa/tawa pona'. There is very little conversational tp and almost none of extended scenarios (The Little Prince, Some short scenes of mine, maybe a couple I have forgotten). We don't know how to talk naturally. (Oh. there may be more on irc records, but those may be artificial and, anyway, seem to be hard to get). This is at the heart of our lack of ways of dealing with common situations, like going to the store or ordering a beer, and of dealing with sentence fragments, which most conversations largely are. We don't even have clear equivalents for "Ma'am" and "Sir" as in your scenario.
Maybe a campaign to get such life events in print -- in the Life-style section of the Forum say -- needs to be launched.