My most expensive fish? Hmm. The two goldfish cost 10 cents each, the bettas all cost between $1.99-2.99, and the CAE cost somewhere around two bucks. So really none of my fish are very extravangant; I even got a white, black, and blue marbled Plakat for under 3 dollars.
However, I guess my tanks, and the MEDS from my "rescues" was the most expensive. All of my bettas - which at one point was up to 12 - have been in 5 gallon filtered tanks with tons of silk plants, gravel, and gaves. Recently I've added a lot of live/frozen foods and different brands of pellets into the mix, as well as blackwater extract. So, I think my combined betta population was quite expensive indeed.
My goldfish have also been pricey, but it was more because I didn't know how to take care of them right when I first bought them, so I started with a 5 gallon tank, then got a 10 when I was told it wasn't big enough, then a 30gal when I realized THAT wasn't big enough either, and now I'm looking into a 50-75 gallon (there are two fish; commons.) I'm sure by the time they are in thier proper tank, they'll have cost my a fortune. I wish I had my own place so I could just hurry up and make a darn pond instead.