i am going to assume that the hot dog combo is the same as Sams club's $1.50 combo. (I don't have a costco near me at all. I live in Walmart country)
Sams runs a 32oz drink with theirs. You could claim that it is only a 16oz with the ice but on the other hand you have free refills so I think 32oz is a fair average between ice and refills.
At that point it is 120 drinks per 5 gallons of syrup. Using your figure of $75 that leaves each drink costing $0.62 plus the cup. (Ice is negligible in cost.)
Non branded cups are $0.075 ea. I doubt branding adds much cost at scale.
Hot dogs are $96 for 120 (40 lb case) That comes out to be $0.80 per.
As for buns it looks like ~$0.15 per bun.
So add up soda + cup + hot dog + bun and you get $1.64 per meal. I left out lids, straws and napkins but those are small amounts.