Most spent in one payment? Lol.
I tend to spend quite a bit to assemble my costumes ( including the lightsabers - whether it's a completed saber hilt or hilt parts) to complete them which means sourcing items from different places plus the shipping costs to get them to me. Though I think the most I've ever spent in one sitting is for a beautifully converted Denix Mauser into a Luke ESB Blaster replica done by Scottjua.
I was originally going to convert it myself, but seeing as he has all the proper equipment to do it, along with the time and experience, I figured it was worth the money to commission it to be done properly.
I want to say I spent about $700-$800 to acquire all the parts and the commission and shipping fees.
Worth every penny! He did an incredible job.