This is a really hard thing for me to keep track of. Most of my costume materials I buy on sale by keeping an eye out for them and I have never really kept track of how much I do spend on each individual costume. Probably because I am terrified of knowing.
If I was to hazard a guess though, my most expensive would be my stargate offworld uniform.
Because I didn't make this costume but instead bought all the parts. While the pants and cap were cheap, I dropped $70 on the boots, $50 on the vest, $50 on the zat, each patch costs $10 and there are at least four and then there was the screen accurate jacket which set me back $250. I warn you now, if you want to put together an offworld uniform, save your pennies!