Thanks for Your Reply - It is obviously not a good thing that You have found Cast Iron Sinks lately to be of a Poor Quality - I am surprised about this, hopefully there will be NO Impurities where You will be Drilling.
When I was Drilling the Cast Iron Soil Stacks - AND the Cast Iron Baths & Cast Iron Sink that I mentioned I was using `Normal` Best Quality High Speed Steel Holesaws [NOT on High Speed for Cast Iron] - For a Tap [Faucet] Hole probably available in the U.S. for no more than $30 [?] - [Depending on Size] - I used to use Holesaws / Hole Cutters manufactured by a Company called `Starret` - I take it that the $150 Bit that You mentioned is the Diamond Tipped Ceramic / Porcelain Hole Cutter ?? - `Reassuringly Expensive` !
When I Drilled the Baths and the One Cast Iron Sink [Years ago] I used a Glass Hole Cutter and a Porcelain Tile Hole Cutter - Then Drilled the Cast Iron with the High Speed Steel Holesaw [NOT on High Speed].
Although the Ceramic Tile Hole Cutters & Glass Hole Cutters were more expensive than the High Speed Steel Holesaws - NOT 5 Times the Price.
I cannot recommend that You use those `Ordinary` Tile Hole Cutters / Glass Hole Cutters in case of an Accident / Damage - BUT apart from the `Professional` Bath Hole Cutting Process those were ALL that was `Readily Available` to even attempt cutting through the Vitreous Enamel Surface `In those Days`.
I would have Drilled through the `Top Surface` with the Glass Hole Cutter - Just breaking the Surface - Then changed to the Ceramic Tile Hole Cutter to Drill to the Cast Iron Surface - Then the High Speed Steel Holesaw [NOT on High Speed for Cast Iron] to Drill / Cut through the Cast Iron - These should be ALL the Same Size - I would have more than one of the High Speed Steel Holesaws [Just in case] - You must use a Light Oil as a Lubricant / Coolant for the Hole Cutters.
I would advise an Electrically Powered Drill - NOT a Cordless Drill - Drilling WILL drain Batteries Quickly - Even Good Quality Batteries.
I wonder what is the Price in the U.S. for the `PORSADRIL` Kit ?? [On the website that Link is to]
I found this webpage online:
How to drill a hole into a steel cast iron bath with PORSADRILL
Although I know that You are fully aware of the Drilling Process I wondered if reading the Details about Drilling a Cast Iron Bath and what was used might be a little Helpful [?].
"Good Luck with Drilling the Cast Iron Sink".