“See.” The Greek word is idou (#2400 ἰδού), and it is used to get our attention. See commentary on Matthew 1:20 (“Look!). Here it is not spoken with great force, but to remind Jesus of the sacrifices the apostles had made.
“…so what will we have?” Peter’s question is a good one, and one that all of us should be asking more. All of us are either like the rich man in Matt. 19:16-22 who hold on to worldly things and lose out on heavenly things, or we are like Peter who has “left everything” and will have great reward in the kingdom.