How much does it cost for an order of fish and chips if you don't get the fish and you hold the chips?

Ignore the fact that only 9 percent of the items on the Fish and Chips menu here include both fish and chips, I'm still trying to figure out the costs. Chips seem to add £1.90 to every other Fish & Chip option (regardless of its fishiness), but if you order chips without the chips, then they only cost £1.90 — yet, if you add chips to your chips (which is presumably one whole order of chips, rather than 2), then it only adds £0.50 to the standard cost that you would otherwise pay for the null set of chips.

Am I doing this right?

Top Image: LearningLark / Flickr (CC 2.0)