。
How did you fall for such a cheap scam?
Literal
Why such cheap-looking swindle [to-に] get-caught [explanatory-んだ].
安っぽい ('cheap-looking') uses the ~っぽい suffix on 安い to mean '-ish, having the quality of cheapness.' ペテン ('swindle, scam') is a loanword from Chinese. ひっかかる ('to get caught, to fall for') is the intransitive of ひっかける ('to trick'). The んだ ending demands an explanation — 'how could you possibly have fallen for this?'