Not sure about the comment of 'lowest end server' - a quick Google gave this Dell T30 as the first result for £406.98 (= $533.76, at least until the end of the month when Brexit hits the economy and the £ crashes), so the price point is similar.
The R240 is a rack server, and there's lots of practical reasons why you want rack-installable units. As for the actual hardware - most servers will use ECC (error-correcting code) memory, which comes at a premium, along with a server-class processor like (Intel) Xeon or (AMD) Epyc. Again there are several reasons, but compared to consumer class equivalents they will have more cache, more PCI Express lanes, support for more RAM, and more cores. The motherboard supporting all this will also be fundamentally different, sometimes supporting multiple processors, larger memory banks, or more expansion ports.
Server Workstations(rack or BOX) are designed to work 24/7, usually have more robust build(you know there s a difference in industrial and consumer grade electronics) and much better cooling than consumer grade PCs/Workstations.
You're essentially asking why people eat a variety of food rather than just consuming a well-balanced vitamin powder - the comparison doesn't start from a proper place.