The installation was spread over a few afternoons, and the cost is hard to break down because I had a lot of the stuff on-hand -- and a lot is also somewhat more expensive because it was higher quality marine (Coast Guard-approved) components (wire, connectors, fuse holders). My one hobby that I'm more involved with than my Volvos is my boats :-). The only thing that was automotive, instead of marine, are the relays and Dan Stern's H4 sockets. But all in all, it couldn't be that much. Let's see:

Ten feet (more than enough) of 10 gauge wire (marine for corrosion resistance, or else simply automotive grade) in two different colors (to not confuse positive and negative cables -- I never use the car's ground, or a boat trailer chassis either, because of corrosion concerns). How much could that cost?

Maybe a dozen marine, crimped connectors (heat shrink wrapped, with adhesive inside for sealing) -- yes, these are a bit costly (can't remember how much, as I bought them in a pkg of 100). Also some like-styled butt connectors to attach things like the fuse holders' pigtails to the main cables, and some like-style ring connectors, too.

And the waterproof, marine fuse (spade-type) holders. Maybe six or seven dollars each (I really can't remember, though).

The two most expensive components were the automotive relays (boats' relays would be really too expensive because of their design for spark suppression); and Dan Stern's high temperature-resistant plugs for the H4 bulbs in the Euroheadlights.

I also had to fabricate a mount for the relays -- a piece of aluminum bolted on by the bolt used to hold the radiator in place (you can see it in the picture, on the side of the radiator's side support) -- this was probably the most time-consuming step.

The next most time-consuming step was simply wiring into the OEM bulb's plugs in a satisfactory (to me) way (1) to use as signals to activate my relays, and yet (2) also keep the original plugs in case I want to change the car back to DOT headlights if, for some reason, I wanted to sell the car later.

Sorry I couldn't be more specific.