When Prince Arragon reads the inscription on the lead casket, he sees the following:
'Who chooseth me must give and hazard all he hath.'
You shall look fairer, ere I give or hazard.
The prince's response is quite ambiguous. He is either addressing Portia or the casket itself. Before he took a look, he referred to lead as 'base', which means that he does not deem it worthy enough. In this sense then, one could interpret that he is referring to the lead casket. He means that it should look better before he is prepared to give or risk all that he has. The casket is not appealing enough for him to do so.
Alternatively, he might be referring to Portia. She is not beautiful enough for him to do what the inscription demands. One could also interpret his statement as a reference to both Portia and the casket. In this regard, his remark is quite hurtful. However, since he is depicted as quite arrogant (hence the play on his name, 'Arragon') it is not an unexpected reaction.
Further evidence of his arrogance is shown when he comments about the writing on the gold casket:
'Who chooseth me shall gain what many men desire'.
The prince sees himself as being a breed apart. He is different and refuses to be deemed common. He interprets the word 'many' as meaning the 'fool multitude' who is easily deceived by gold's false glitter. Moreover, he
... will not jump with common spirits
And rank me with the barbarous multitudes.
as previously mentioned.
It is ironic that the prince chooses the silver casket for two reasons. Firstly, the inscription promises:
'Who chooseth me shall get as much as he deserves:'
The prince believes that he deserves the best and in his vanity assumes that that is definitely what the casket holds. He deserves Portia and is confident that she is what he will get. He is sorely mistaken, however, for the casket renders a fool's head and the inscription inside states that the idiot who had chosen the casket would now not only have one fool's head but two.
It is ironic that the prince had earlier stated that he will assume 'desert' for he had just gotten his 'just desserts.' He was rightly rewarded for his arrogance and conceit and was given exactly what was his due.