I don't get this scene in Keeping the Faith...

Father Brian Kilkenney Finn: Is this a good machine?
Don: [In heavy Oriental accent] Yeah, that one good. If you a cheap bastard! No, that one OK, but if you serious about car - OK, then there only one machine for you. The Audio 2000! This baby got multiple inputs, dual pitch analyzer, so you can change the pitch, if your voice sucks, but I don' need that.
Rabbi Jacob "Jake" Schram: How much does it cost?
Don: [shouts] Price is not important
Father Brian Kilkenney Finn: No, price is very important.
Don: OK, OK, you got me, take me away. It's a little bit expensive, but it's worth it. When you sing, to your girlfriend, and her heart waboom, fall down on the floor, you say Thank you, Don!
Father Brian Kilkenney Finn: How much... does it cost... exactly?
Don: [looks around and speaks in ordinary American accent] Ok, I don't usually do this but you guys seem like a couple of cool guys and I got a nice piece of ass last night. I'm gonna let you have it for $1300.
Father Brian Kilkenney Finn: $1300? Wow. Sorry I'm getting a little hot.
[unzips his jacket to show his priest's collar]
Rabbi Jacob "Jake" Schram: My friend gets a little hot sometimes.
Don: [In normal voice] Oh, man! What is that? Get out of here with that! Is that real?
Father Brian Kilkenney Finn: Oh, yeah.

What am I missing that I don't get that last part, what would Brian being a priest change?

I would assume from this quote that the people selling the item are religious and he hopes to get a lower price because he is a priest.

I haven't seen that films since it was in cinemas... so I can't say i remember it too well.

What am I missing that I don't get that last part, what would Brian being a priest change?

Charlie_Weasley

I think that the salesman was trying to get more money out of them, so when he realized that Brian's a priest, he decided that he shouldn't scam a priest, a.k.a. a holy man of God.