I think that, spoiling her with toys and chew things aside, my chinchilla is one of my least expensive critters. I do have her on Carefresh, mostly because it is cleaner and I know that fleece liners would never stay put with her, so that's probably the most expensive thing right there. The price on that stuff has really jumped! I think it's like $15 or so for the big bag, and I go through about one a month between the chin and two hamsters.
Electricity is another factor. In my old apartment, where we were on the top floor, we were spending an extra $80 on air conditioning. This is in a STUDIO apartment, mind you. We would have had it running just as much without the chin, since neither the boyfriend nor myself can tolerate very warm temperatures, but it is still something to keep in mind.
And as for vet costs, I haven't had to take my chin to the vet in the two years I've had her (thankfully!) but I took a guinea pig in last March and my vet charges the same rate for all rodent-type critters (which I think is around $70 for the visit plus extra for treatments). He and his cagemate got in a fight and his lip got split open. To have him stitched up, which required anesthesia, and medicated to prevent infection, cost around $250. So, just be sure you're able to pay vet bills if necessary. If they are just pet chins (not breeders), having an extra $500 available is always a good thing.
The expensive thing is always the initial cost. Cages, toys, and even the chin itself can cost a lot! I have a wonderful metal ferret cage I found on sale at Petco that set me back $150. Sure, it's big enough for three or four chins and I only have one, but what can I say? My Zumi is a spoiled little princess
Once you're got your chin all settled in, though, it's not bad at all! A small bag of Mazuri ($7-$8) lasts me over two months and when I bought the small bags of hay (now, with four guinea pigs and two rabbits, we buy the 50 lb box from Oxbow) that would last me a month, and I think those run about $5 now. Oh, and I can't forget the cost of the organic raisins from the co-op, haha
A small bag, around $4, lasts like... well, probably six months, since she only gets one a day (and no more than that, no matter how much she begs!)
So, compared to our four male guinea pigs, who go through a $15 bag of food and around 10 lbs of hay every month, not to mention the $15 or so we would spend on greens every month, AND the cost of washing their fleece and towels every five days ($1.25 to wash, $1.25 to dry, and now that the cage is bigger it takes two loads)... Chins are VERY cheap