If the dealer is ordering the booth from the manufacturer then there's shipping +the booth cost. $500 is pretty cheap.
JMO but go sit in a similar booth for 45 mins and see if your butt hits the plywood support under it. I've never found a booth that I could sit in for more than 30 mins. I'm not heavy either. Actually a light weight. Most foam used in booths is cheap.
99% of the time a free standing dinette is an option for more money. I would negotiate a deal with the dealer. He should give you something for your FS table. They can sell that to someone. Most RV shops I've been around have a back room with these things.