Let's see...
$19 for the pair of 90v power supplies
$3.50 for a single 12v power supply
$2.50 for a pack of piggyback connectors from RadioShack, because I was too impatient to order online (though I have 7 left over)
$1.25 for two diodes from RadioShack
$0 for a ring terminal and two splice connectors; already had a big box of connectors
$0 for a power supply I found in the garbage at my university, which already had a fuse, fan, and some nice connectors.
$0 for a spool of solder and a nice temperature controlled iron; already had
$0 for heat shrink tubing; already had
$0 for some speaker wire; already had
$0 for some zip ties; already had
$0 for electrical tap; already had
$0 for wire cutter/stripper
I guess if you were starting with nothing, a nice iron will run you ~$30, and you'd need to buy a few large boxes of connectors, might come out to $30 total for the items I listed as "$0", but you'd have enough spare to make 20 grid chargers. I'm always building small electronics and have bins and bins of junk for use as parts, so it's hard for me to know what the actual cost to make things is.