Rod, I'm not sure where you're shopping, but internal drives are usually (still) cheaper than
comparable external drives. Every once in a while, they are equal in price, or slightly lower with short-term sales, but that isn't the rule, and we're usually talking by a couple bucks.
While the channel was limited, though (post-flood) the reverse was true for a little while. That's because channel pricing reflects demand and availability. The OEMs (Dell, ASUS, Apple, HP, etc) were sucking up almost all of the internal drive capacity available to stick in their machines, so retail prices for bare drives were higher. OEMs don't sell huge numbers of external drives directly, so those prices were more "fairly distributed".
Now, if you are comparing a 2TB WD Black 7200RPM high-end drive to a 2TB external, then sure. But that external drive isn't using a 7200RPM high-end SATA drive inside. It has the
cheapest "green" drive they can buy inside.
Prices:
Seagate 1TB USB3 Drive: $99 ($79 on sale right now) - cheapest 1TB external drive on Newegg today
Hitachi 1TB 7200RPM SATA drive: $79.00 (regular price) - cheapest 1TB internal on Newegg today
Samsung 2TB USB2 Drive: $129 ($99 on sale right now) - cheapest 2TB external
WD 2TB Caviar Green: $119 - cheapest 2TB internal
Seagate 3TB USB3 Drive: $159 ($129 on sale right now) - cheapest 3TB external
Seagate 3TB 7200RPM SATA: $159 - cheapest 3TB internal
So, they're basically even, except for occasional short-term sales (which can go either way, there were sales last week on the 2TB Greens that brought them down to $89). Also, for "low capacity drives" the pricing is all crazy, so you can't really go by anything. Anything below about 750GB isn't really being manufactured in any quantity anymore, except for special OEM usage. Stuff still available is almost entirely overstock from the OEM market, or left-overs from when they were still being manufactured. Pricing on that is all nuts, though, and can vary wildly from vendor to vendor.
Power supplies in the external drives all universally SUCK. They're not worth buying, in almost any case. You'd always be better off getting a good internal drive, and
one of these.